Biology 30
Module 3
Reproduction and Genetics
Lesson 11
Types of Reproduction and Development:
Single Organisms to Vertebrates
Copyright: Ministry of Education, Saskatchewan
May be reproduced for educational purposes
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Lesson 11
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Lesson 11
Lesson 11 Reproduction: Cells and
Organisms
Directions for completing the lesson:
Text References for suggested reading:

Read BSCS Biology 8th edition
Sections 6.6-6.8 7.3-7.6
Pages 133-139, 146-152
OR
Nelson Biology
Pages 509-531, 563-564


Study the instructional portion of the lesson.


Review the vocabulary list.

Do Assignment 11.
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Lesson 11
Vocabulary
amniocentisis
amniotic egg
blastocyst
chorion
chorionic villus biopsy
epididymis
estrogen
external fertilization
feedback
fetus
FSH
germ layers
gestation
internal fertilization
LH
menopause
menstruation
morula
ovaries
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oviducts
oviparous
ovoviviparous
ovulation
progesterone
semen
teratogens
testes
testosterone
tuballigation
ultrasound
umbilical cord
uterus
vagina
vasdeferens
vasectomy
viviparous
yolk sac
zygote
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Lesson 11
Lesson 11 – Reproduction and
Development of
Vertebrates
Introduction
In plants and other organisms below the vertebrate level, reproduction of new
individuals is commonly accomplished by asexual methods or a combination of
asexual and sexual phases. This last form is known as alternation of generations. In
the vertebrate groups, the haploid (n) phase is represented only by gametes (eggs and
sperm). There is neither multicellular development of haploid cells nor a
gametophyte stage which eventually produces gametes. For this reason, vertebrates
are considered to lack alternation of generations and to reproduce entirely by the
sexual method. This is also evident in some other taxonomic groups, such as
arthropods.
Relying almost exclusively on sexual reproduction to produce new organisms, species
also developed the characteristic of individuals being female or male, but not both.
While hermaphrodites are common in less complex multicellular organisms, they
are not numerous in arthropods or vertebrates. Appearances of hermaphrodites in
these groups tend to be accompanied by physical (structural) and behavioral
abnormalities.
The development of separate sexes in sexually reproducing species introduced a
number of difficulties. Some of these relate to actions of trying to successfully bring
eggs and sperm close enough together so that fertilization could occur. After
fertilization, other possible difficulties arise. These have to do with the successful
development of a zygote into an independently functioning individual. This lesson
will look at reproduction as it applies to vertebrates, from pre-fertilization to the
hatching or birth of young and what conditions face the young of various groups.
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Lesson 11
After completing this lesson you should be able to:
•
identify some of the difficulties of organisms in carrying out
successful sexual reproduction and conditions which can
increase the success rates of external and internal fertilization.
•
explain some of the different adaptations among species which
increase the chances of survival for the young after they are
hatched or born.
•
discuss some pre-fertilization conditions and their significance.
•
name and explain the four major ways in which embryos could
develop among different groups.
•
compare amniotic egg of birds and reptiles with the structures
that form in the uterus of a pregnant mammal.
•
describe some of the features related to viviparous
reproduction, with emphasis on humans. In particular,
·
·
·
·
·
female and male structural differences.
female reproductive cycles.
production of semen
the fertilization process.
the establishment of an embryo inside the uterus.
•
summarize some of the stages in embryological and fetal
development and the general birth process most placental
females go through.
•
discuss the biofeedback mechanisms involved in the
reproductive cycle.
•
discuss the relationship between diet and health of mother and
the development of the fetus.
•
describe how the use of hormones in birth control pills alter the
reproductive cycle.
•
discuss some other aspects related to human reproduction,
such as
· birth control
· amniocentesis
· ultrasound
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· sexual diseases
· fertility drugs
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· genetic screening
· in vitro fertilization
Lesson 11
Requirements for Successful Sexual Reproduction
In biological terms, the sexual process relates specifically to an action where two
reproductive cells or gametes fuse together. These gametes, haploid in number,
combine to form a diploid zygote which can then undergo mitotic divisions and
differentiations in forming another organism.
One of the initial requirements of successful reproduction is to bring the different
gametes close enough together so that the fusion process can take place. Critical to
this is the fact that individual eggs and sperm often do not have long survival times.
In the process of oogenesis, the first meiotic division produces a large cell and a
smaller polar cell or body. The smaller polar body may or may not disintegrate
immediately. The next meiotic division involving the larger cell again results in a
large cell-small cell combination. The larger cell remains as the egg. Having more
cytoplasm from the unequal divisions, this cell or egg can sustain itself for a longer
period of time. However, in most instances, this "longer" period of time is still only a
few days.
Spermatogenesis goes through similar division processes, except that four,
equally-sized cells or sperm result. This leaves each sperm with very little cytoplasm
so it is incapable of sustaining the cell for very long. Sperm develop flagella and are
capable of swimming movements. This energy-draining action contributes to the very
short life spans for the sperm of vertebrates. These characteristics of eggs and sperm
therefore make it important that the formation and release of the two types of
gametes be timed so that they occur almost together. Also important is that the
sperm be placed close enough to the eggs so that they may meet quickly.
A zygote, resulting from a successful fertilization, is the beginning or the initial cell
of a new organism. As with any other organism, basic processes and requirements
must be satisfied if life is to continue for the zygote and then the embryo. The other
major requirements for successful reproduction include:


the provisions of a suitable environment
a sufficient source of nourishment for the developing embryo until it is ready for
hatching or birth.
The degree to which different species satisfy or meet these conditions is reflected by
the mortality rates of the embryos and the very young of those species.
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Lesson 11
External-Internal Fertilizations
For less complex, multicellular organisms, the sexual structures or organs involved
in reproduction generally experience only limited developments. The structures
could include just the sexual organs or gonads (ovaries and testes) and oviducts and
sperm ducts leading from these to the body surfaces. These limited developments are
carried on into a number of taxonomic groups and extend into the vertebrate level.
Initial reproductive actions could be either one of two processes.
1. Both eggs and sperm can be released to the outside where fertilization may occur;
or
2. Eggs can be retained in the body cavities or chambers of one individual and can
be fertilized by sperm released from another (as in clams).
Water plays a very important role in external fertilization. Unless a sperm cell is
directly deposited onto the surface of an egg, there must be a way for it to reach the
egg. The method usually involves a fluid medium, which enables the flagellated
sperm to swim about until a chance meeting with an egg may occur.
It can be rightly assumed that external fertilization is not a very efficient
process in bringing an egg and a sperm together.
Success rates of just some fertilizations happening increase with the production of
very high numbers of eggs or sperm, or both. Such a characteristic is quite common
among externally fertilizing species. For instance, each spawning season, fresh and
salt water fish can produce eggs or sperm which can number in the millions. Despite
this, the reproductive rate can still be quite low. Many eggs are not fertilized, while
those which are could be eaten by other organisms or fail to develop in unfavorable
conditions. Even after hatching, predation rates upon the young are high.
The chances of eggs being fertilized increase as sperm are deposited closer to the eggs.
The spawning actions of many fish have females and males side by side or the male
following the female so that sperm is released at almost the same time and as close to
the eggs as possible.
In some amphibians, such as frogs and toads, a male will sometimes grasp the
female from the back in an action called amplexus. This stimulates an egg laying
action in the female. As the eggs are released from the female's cloaca, the male
discharges sperm over them. Water is still needed as a transporting agent and this is
one of the reasons why amphibians must still return to water to reproduce.
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Lesson 11
Closer contacts eventually see adaptations in species where males actually transfer
sperm into females' bodies. Any action of this type is part of internal fertilization.
The "simplest" actions have two individuals coming together and lining their bodies
up so that certain body openings are opposite each other. Hermaphroditic
earthworms carry out this action so that one individual releases sperm which enter a
special cavity or seminal receptacle of its partner, while receiving a transfer of sperm
into its own seminal receptacle in return. In other species, a male and female line up
to carry out a similar action of sperm transfer from opening to opening. Slight body
grooves could serve as pathways in some instances.
Some species (spiders) have males using their limbs or walking legs to transfer sperm
from themselves to the openings of seminal receptacles in females.
In many insects, females and males bring the ends of their abdomens (and
abdominal openings) together, allowing for direct sperm transfer to a seminal
receptacle. In some situations, sperm can be kept alive in females for months and
are used at whatever times that eggs are in the process of being laid.
In birds and reptiles, close contact between female and male cloaca permit sperm to
pass from one to the other. In these individual groups, sperm are used almost
immediately for fertilization as they work their way to the oviducts, rather than being
stored.
In mammals, further developments exist in addition to the gonads and ducts for egg
and sperm production and transfer. Males have special copulatory organs for
inserting into females' bodies and depositing sperm. This method of carrying out
internal fertilization makes it more likely that a high number of sperm will have been
internally deposited, closer to the eggs.
Although internal fertilization does take place in some aquatic species,
its greatest benefit is to land or terrestrial organisms. Without the
medium of water to be used for sperm transfer from one individual to
another, external fertilization would not be able to take place. On land,
this difficulty is overcome by placing sperm on the surface or inside the
body of a female. Even though water is not as important for internal
fertilization, fluids still play a part in the process. Body fluids of males
and females still provide a means for sperm and eggs to move about
inside reproductive canals.
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Lesson 11
Pre-Fertilization
What has been mentioned previously should have given an indication that the
success of fertilization depends very much on two things. These are:
1. timing in the formation and release of eggs and sperm and
2. having the gametes placed as closely together as possible.
Both of these conditions are necessary because of the gamete cells' short life spans,
which may average only several days. It is therefore important that females and
males find each other and carry out mating behaviors at the proper times.
A number of different factors could have influences in bringing individuals together
and initiating reproductive behaviors.
Hormones play a large part in reproductive cycles. For many species, hormone
levels themselves could fluctuate according to photoperiods. Certain periods of light
and darkness over a length of time can affect the hypothalmus of the brain and the
pituitary gland. These then either increase or suppress the levels of certain
hormones which affect the reproductive organs. Females and males exposed to the
same environmental conditions should experience body changes which tend to match
each others' in terms of levels of development or where they are in the reproductive
cycle. Photoperiods and hormone levels set up distinctive annual cycles for many
species.
Examples:

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


The male members of the deer and goat families go into an autumn rut. This is a
reproductive behavior in which they establish territories, actively seek out females
and try to establish dominance over other males by rituals or by fighting.
The annual reproductive behavior for waterfowl and other birds takes place in the
spring.
In other species, a number of reproductive cycles could take place over certain
time periods. Mice could produce two to four litters from spring to fall.
Just as changes in hormone levels initiate reproductive behavior at certain times,
they also cause it to decline and stop. Many species will even stop producing eggs
and sperm and are, in effect, sterile for certain periods of time. Male deer or
bucks, at times other than the fall rut, are usually in this condition. Fish and
many birds also experience this after spring spawning or egg laying.
many domesticated livestock and pets have lost much of the behaviors to breed
only at certain times of the year. Horses and cattle can breed and reproduce at
any time of the year, although there is still a tendency to follow a certain yearly
pattern.
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Lesson 11
Courtship behaviors often arise as a result of certain hormone levels. Largely
inherited or innate, these behaviors can be used to find possible mates at a
reproductive stage where breedings can occur. Songs of male birds are not only
intended to warn other rivals, but are used to attract females. Displays of brightly
colored plumage, "dancing", head bobbing or other actions can all do the same thing.
The importance of courtship behaviors to successful fertilization is especially
noticeable in some groups which follow very ritualistic behaviors. Individual pairs of
some fish, birds and other organisms have series of responses and counter responses
made by females and males. If one of the partners is not reproductively ready and
fails to carry out a particular action in a sequence, the entire courtship ritual usually
stops at that point. The other partner may start all over again at the beginning or
lose interest and move away. Should a female and male be at a stage where egg(s)
and sperm are fully developed and ready for fertilization, it is more likely that a
proper sequence of actions would be followed through. This would be culminated by
a mating or spawning process where fertilization could occur.
Many courtship behaviors are based on vision. Displays are intended to be seen by
other individuals and reacted to. The kinds of reactions made would determine the
sequence of events to follow. But sight is just one of a number of possible senses
which could be important to reproductive processes. Pheromones, or chemical
"messengers" released from bodies, are used to attract potential mates through their
sense of smell. Females of large numbers of species, in a state of reproductive
readiness, release chemicals or body odors which attract males. Male odors could
also stimulate some females and initiate or accelerate their reproductive behaviors.
Sounds and hearing serve as other forms of communication between individuals.
Even touch or contact can be used as a means to either attract or repulse other
members of a species.
It should be emphasized again that all of the conditions mentioned to this point, as
well as others, could play a part in bringing females and males and then eggs and
sperm together. A successful fertilization is one of the first steps to a possibly
successful reproductive process.
Variations in Fertilization and Embryo Development
Vertebrate reproduction can follow one of four major patterns:




External Fertilization
Internal Fertilization – Oviparous
Internal Fertilization – Ovoviviparous
Internal Fertilization – Viviparous
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Lesson 11
External Fertilization – Aquatic Development
External fertilization is carried out primarily by fish and amphibians. Their eggs are
fertilized externally almost simultaneously as they are being laid. A female and male
are usually close together when these actions are taking place. The eggs often have a
surrounding protein material which soon absorbs water and swells to form a jellylike
coating. As well as holding the eggs together, this coating helps to protect the eggs
and also helps to maintain a more stable temperature around them. A supply of yolk
in each egg sustains the developing embryo for the one to three week time period it
normally takes from egg laying to hatching. Aquatic development is necessary for
such eggs as an exposure to air would dry and kill them. In addition, respiration by
means of gills is common to most species – at least for part of their life cycles.
Internal Fertilization – Oviparous Development (Amniotic Eggs)
Organisms which lay eggs for external incubation are known as oviparous.
Depositing eggs for incubation on land could happen only when the eggs are covered
with shells relatively impermeable to water, but still allowing gases to pass through.
This would permit an embryo to develop within an enclosed, watery environment,
despite being exposed to air. Such an adaptation appeared in birds, reptiles, the
duckbill platypus and spiny anteater. However, enclosing an egg in a leathery or
hard shell not only stops the movement of water, but also prevents the passage of
sperm. Therefore, these groups must experience internal fertilization before the
egg shell is formed. For the majority of organisms, sperm transfer is accomplished
when females and males come together and establish contact with their cloaca.
Cartilaginous fish, such as sharks, also carry out internal fertilization. Sperm
transfer occurs along the grooves of fin-like claspers of a male shark to the female.
Once an egg has been fertilized near the upper end of an oviduct, it begins moving
down. Modified regions along the oviducts then form the shells around the eggs as
they move through. In birds, the process from fertilization to when the egg is actually
laid could take three or four days.
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Lesson 11
 Shelled eggs show the first appearance of membranes important to embryo
development. Just inside the shell is the chorion. The chorion is a membrane
richly supplied with blood vessels. This is where exchanges of oxygen and
carbon-dioxide between the shell and the embryo occur. Also present is a
membrane around the embryo called the amnion. The amnion contains the
amniotic fluid which serves as a protective cushioning agent for the developing
embryo. Also contained within the chorion is the allantois and yolk sac. The
allantois, like the chorion, is also involved in gas exchanges with the embryo.
Another function is to serve as an enclosure for waste material from the embryo.
These toxic wastes are largely converted to insoluble, less harmful uric acid which
is then held within the membrane. The yolk sac membrane contains the supply of
food necessary to sustain the developing individual until hatching and, in some
instances, for a short time after.



In summary, an amniotic egg with its membranes has a number of ways of
increasing the success of land reproduction.
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

An external shell reduces or stops the loss of water, preventing an embryo from
drying out.
The nature of the shell and membranes enables gas exchanges to occur between
an embryo and the outside.
A supply of food enclosed within membranes serves to nourish the developing
embryo.
Membranes also remove and store its wastes.
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Lesson 11
Internal Fertilization – Ovoviviparous Development
Organisms which fertilize eggs internally and retain the eggs within their body until
after hatching are Ovoviviparous.
Ovoviviparous development is prominent in some fish and reptiles (guppies, garter
snakes...). The eggs are fertilized internally and have membranes or shells formed
around them, and are retained in the females’ body until after hatching. The hatched
young are then expelled from the mothers' bodies. From the time that eggs are
fertilized to the time that the young are born, a mother's only role is that of providing
a more stable and secure environment for them. Nourishment for a developing
embryo comes from stored yolk within the egg. Most young are independent and
go their separate ways right after birth. In some species, young have to be
particularly careful as their own mothers could turn cannibalistic and devour them.
Internal Fertilization – Viviparous Development
Internal fertilization and incubation, with dependence on the mother after birth is
Viviparous development.
Most mammals give birth to their young after they have experienced internal
development with some degree of nourishment provided by a mother's body. This
form of reproduction is characteristic of marsupials and placentals.
Among the marsupials, the period of internal development can range from 8 to 40
days for various groups. Most of the nourishment comes from a small quantity of
yolk. A yolk sac partially embeds into the lining of a mother's uterus to provide for
some movements of nutrients and wastes between mother and embryo. Development
is still very incomplete when a young organism is born. After birth, it must make its
own way through the mother's fur to her marsupium or pouch and, once inside,
fasten onto a nipple. Development is completed inside the marsupium. The
particularly difficult journey from birth canal to marsupium and then a somewhat
precarious existence inside the pouch places marsupials at a reproductive
disadvantage when compared to placentals.
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Lesson 11
Initial embryonic beginnings of marsupials and placentals are similar. The manner
in which placental embryos finish their developments gives this group a decided
survival advantage over marsupials. In placentals, embryos initially have only a very
limited amount of yolk material which is quickly used up. However, the membranes
of the yolk sac and allantois (amnion) are modified so that they form a connection
between an embryo and the uterine lining of the mother. This connection is called
the umbilical cord and the area where the end of it embeds into a portion of the
uterus is the placenta. Blood vessels from the mother and the embryo are close to
each other in the placenta, but there is no direct connection between the two
circulatory systems. Nutrients and wastes pass between them by diffusion through
the lymph between the capillary systems.
The placental connection enables a young individual to be fairly well formed at birth,
after a longer period of internal development.
The membranes in mammals are similar to those found in the amniotic egg of birds
and reptiles.
Insert image comparing membranes of birds and humans if desired.
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Lesson 11
Viviparous Reproduction – A Closer Look at
Placentals
The dominance of embryo development with a placental connection among mammals
warrants a closer examination of this reproductive group. Although many of the
references and descriptions will be made to humans, it should be kept in mind that
structures and processes are very similar among placentals in general and that
references could just as easily be made to other specific groups.
Female-Male Structural Differences
Female Reproductive System
The major structures in a placental female's reproductive system are a pair of
ovaries, oviducts (also called fallopian tubes) leading from these to an enlarged
uterus, and a canal or vagina leading from this to the outside.
Image by Conruyt
A separate tube, the urethra, leads from the bladder to a separate opening just in
front of the vaginal opening. In primates and some other placentals, the single
uterus could be considered as an enlargement and then joining of the two fallopian
tubes or oviducts. In other placentals, especially where multiple young are common
(as in rodents), the enlargements of the two oviducts or uterine tubes remain
separate until close to the terminal end or vagina. Embryos could develop in both of
the uterine tubes or "horns" in such organisms.
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Lesson 11
Male Reproductive System
In males, a pair of testes has many seminiferous tubules that are lined with cells
that undergo meiosis to produce sperm. (The production of sperm is referred to as
spermatogenesis.) These tubules lead to a coiled mass, the epididymis (contained in
the testes), where sperm cells mature and are stored until ejaculation or release
occurs.
image by Xiong Chiamiov
Just as in a female, a duct network is important in the transfer of the sperm, or male
gametes. Upon release, the sperm move out of the epidiymis, and into the
vas deferens tube. This tube circles near the bladder and then unites with the
urethra before continuing as a single tube which transports both urine or sperm to
the outside. Along the way, two seminal vesicles, a pair of Cowper's glands (also
called Bulbourethral gland) and a prostate gland, all add fluids to the duct network.
These fluids, along with the sperm, form the semen which a male ejaculates or
releases during mating. Ejaculation occurs from a penis, a structure containing
three cylindrical bodies of spongy tissue located along the sides and top. There are
many blood vessels in this spongy tissue and when the vessels dilate, blood enters
the penis. As this happens, pressure increases on veins leading out and this slows or
stops the movement of blood out. The two combined actions cause the penis to
lengthen and become firm enough to penetrate the vagina so that ejaculation can
occur within the female body.
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Lesson 11
Structural Differences
The male duct network has two major differences from those of females
1. In females, the network is open in that oviducts are not directly connected to the
ovaries, but are separated by an open space. The male ducts are closed or
complete from the testes to the body surface opening.
2. In females, the urinary and reproductive canals remain separate from one
another and have their own separate external openings. However in males, the
urinary and reproductive canals are united, having one external opening.
Hormonal control of Reproductive Cycles
Hormone productions and concentrations are responsible for establishing female
reproductive cycles. For many placental females which are reproductive during only
part of a year, the same day-night characteristics or photoperiods which affect the
males also affect them. This increases the likelihood of both sexes being fertile in the
same time period.
The hypothalmus of the brain and the pituitary gland are the major agents setting
up a reproductive cycle. The hypothalmus directs the pituitary in its production of
FSH (follicle-stimulating hormone) and LH (luteinizing hormone). These two
hormones are responsible for the production of eggs and sperm.
In a male, after puberty, these hormones (FSH and LH) are responsible for the
formation and release of the steroid hormone, testosterone. Testosterone promotes
sperm production in the testes and the development of secondary sex characteristics.
As well, testosterone accelerates or promotes the development of extra muscle tissue.
This last effect is a reason some athletes have used this steroid.
In a female, FSH initiates the maturation of an ovum (egg cell) inside a fluid-filled sac
or a follicle in the ovary. FSH & LH stimulate the follicle to secrete estrogen which
in turn causes an increased blood supply and a build-up in the lining of the uterus.
This continues for approximately 14 days. Just before the 14th day there is a sudden
increase (caused by positive feedback of estrogen on the pituitary and hypothalamus)
in LH causing the follicle to burst and the ovum to be released. This is called
ovulation.
LH stimulates the ruptured follicle to develop into the corpus luteum. The corpus
luteum releases estrogen (stimulated by FSH) and progesterone (stimulated by LH).
(It becomes a temporary gland releasing hormones.) Progesterone acts to cause more
build-up in the uterine lining.
The increased levels of estrogen and progesterone create negative feedback on the
hypothalmus and the pituitary so that FSH and LH levels fall. This stops
development of other follicles. If fertilization does not occur, the corpus luteum
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Lesson 11
begins to disintegrate and the levels of estrogen and progesterone drop. This
decreases the blood supply to the uterine lining and menstruation begins. If the egg
is fertilized and does establish itself in the uterus, developing membranes secrete
hormones which maintain the uterine lining.
Female Hormone Cycle
INSERT DIAGRM OF
HORMONE CYCLE HERE
The female reproductive cycle is an excellent example of feedback regulating certain
body conditions and trying to maintain a balance according to particular situations.
Both positive feedback (where a particular action results in an effect which further
intensifies the action of the first.) and negative feedback (where the product of one
action will begin to suppress the action of the first) are demonstrated in the female
reproductive cycle.
For many hoofed placentals, such as deer, elk, the female reproductive cycle repeats
about once a month during the breeding season. In some rodents such as rats and
mice, it can repeat every few days, unless an impregnation takes place. In other
species, the cycle may repeat in 2 to 4 weeks, but it may only happen two or three
times in a limited breeding season. Many female placentals will show a marked
change in behavior around the time of ovulation, when an egg can be fertilized. In
this period of sexual excitement or estrous, a female may actively seek out a mate
and initiate breeding behavior.
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Lesson 11
Primate females (including human) experience regular reproductive cycles throughout
the year. These menstrual cycles keep repeating unless interrupted by pregnancy.
The human menstrual cycle is shown in the following illustration. It is based on 28
days, which is common to many individuals. However, it should be kept in mind that
cycles can vary from 21 days to 48 days for others. Even an individual can
experience fluctuations in her cycles.
The Menstrual Cycle - Centre
Interactions among the nervous
system and several organs,
glands, and hormones regulate
the cycle.
Note the graph of the rise and
fall of hormone levels in the
regular monthly cycle. (Should
be familiar with this.)
*
follicular phase – The
development of the follicle
occurs. Lasts from Day 6 to
14.
** luteal phase – occurs from
day 15-28 of the menstrual
cycle. The corpus luteum
develops. It produces
progesterone which causes
the buildup of the uterine
lining.
By
Lyrl
Ovulation or egg release in many human females takes place about midway through
a menstrual cycle – about day 13 or 14, with a possible variation of a couple of days
either way. A sharp increase in LH (positive feedback of estrogen) appears to initiate
the release of an egg from a follicle. If fertilization does not occur within 12 to 48
hours in the fallopian tube, the egg will degenerate and be resorbed by the body.
Menstruation will follow.
With age there are fewer functioning follicles, leading to a decrease in progesterone
and estrogen. This decrease leads to menopause, and ovulation and menstruation
cease to occur.
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Lesson 11
The Fertilization Process
Some of the different factors which could lead up to a mating action were described
in an earlier section. Hormones play a major part. However, different senses such as
sight and smell, and also various behavioral actions by a female and male could
influence the timing of a mating action. When a female is receptive, a male will
deposit anywhere from millions to billions of sperm into the vagina at each
ejaculation of semen.
The seminal fluid from the seminal vesicles, Cowper's glands and prostate gland in
which the sperm are mixed serves a number of purposes:



It actively transports the sperm upon initial ejaculation and later it provides a
medium in which the flagellated sperm can swim.
In many females, the vaginal and uterine environments are quite acidic, which is
harmful to sperm. Seminal fluids lower this acidity and give the sperm a better
survival chance.
Seminal fluid has a sugar base and this also prolongs the lives of sperm by
supplying them with some energy.
Despite the high numbers, the mortality rate among sperm cells is extremely high.
All sperm may be dead in less than a day. On the other hand, some could survive
up to five or six days in the female reproductive tract. The cervix is a major obstacle
to many swimming sperm. This is a narrowing between the uterus and the vaginal
canal. Many sperm fail to get past this point. Some female bodies could produce an
extra amount of mucus which accumulates in this area and stops sperm movement.
Those sperm which do get past the cervix continue swimming in the uterus and into
the oviducts or fallopian tubes.
Ovulation of one or more eggs from an ovary, or both ovaries, does not release them
directly into the oviducts or fallopian tubes. A space between the fallopian tube and
ovary requires that eggs somehow be drawn into the funnel-like opening of the tube.
This is accomplished by cilial action and fluid movements. It is in the fallopian tube
that an egg is usually fertilized. The process must take place within several days of
the egg entering the tube, otherwise it will degenerate. Fertilization in humans is
most frequent when sperm enter the fallopian tubes within a time span of anywhere
from 2 days before, to a day after, ovulation. An egg in a fallopian tube could be
fertilized in as short a time as 15 minutes from the moment that ejaculation of semen
into a female's body takes place. The limited survival times of both sperm and egg
cells make the timing of actual mating actions very important in determining
reproductive success.
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Most eggs are fertilized when they are located at the top, or near the beginning, of the
oviduct or fallopian tube. Each egg has a protective membrane around it. An
enzyme carried by the cap at the top of the sperm cells is needed to help break down
this membrane to permit entrance. Once a single sperm has entered the egg, a
chemical barrier to all other sperm is established. The sperm which was successful
in entering the egg loses its tail and combines its chromosomes with those of the egg.
This new cell, with the normal diploid chromosome number, is the zygote.
Establishment of an Embryo in the Uterus
Successful fertilization does not necessarily ensure successful
reproduction. A fertilized egg or zygote takes about two to
three days to move the rest of the way through an oviduct
before entering the uterus. During the movement, the zygote
has been dividing mitotically and also forming a chorion
membrane around itself. This membrane produces and releases
enzymes which help clear the way to the uterus. In the
uterus, the enzymes then enable the embryo to work into
the thickened uterine wall. A successful implantation of the
embryo into the uterine wall means that a successful
pregnancy will have been established.
Embryology and Fetal Development
Almost as soon as an egg is fertilized to become a 2n zygote, mitotic divisions and
cleavage begin dividing it into a mass of smaller cells. These mitotic divisions go on
as the fertilized body makes its way along the fallopian tube to the uterus. When the
embryo reaches the uterus and is a mass of approximately 60 to 80 cells, it is called
a morula. Some variations exist among species after this, with respect to the cell
numbers, cell arrangements and yolk content.
In general, the next stage produces a rounded
layer of cells with a hollow interior. This is the
blastula or blastocyst (as it is more commonly
called in mammals). Implantation into the
uterine wall begins to occur sometime around
this stage.
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INSERT DIAGRAM HERE
Development from Ovulation through Implantation
As the embryo embeds itself into the uterine wall, its cells continue to divide, with
part of the cell layer folding inward. This marks the beginning of a gastrula stage, in
which an embryo develops distinct germ layers: ecotoderm, endoderm and
mesoderm.
A third layer of cells forms when cells split off near the junctures of the ectoderm and
endoderm, to form the mesoderm.
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The three germ layers which result are significant in that each will produce, or
differentiate into, the different kinds of tissues and organs which will make up a
body.



Ectoderm cells differentiate into skin cells, nerves, brain and spinal cord.
Endoderm gives rise to digestive structures.
The mesoderm produces muscles, bones and the major body organs.
From the blastula stage onward, other developments take
place. One of the more important of these is the
development of extraembryonic membranes and, in
particular, the placenta and umbilical cord linking the
embryo to the uterine lining. The placenta, as part of its
functions, acts as a temporary endocrine gland. Soon after it
begins forming, it starts to release (human) chorionic
gonadotropin hormone or HCG. This hormone stimulates
the corpus luteum and its production of progesterone
which, in turn, helps to maintain the uterus and pregnancy.
The early presence of HCG is
the basis of a home
pregnancy test. A kit can be
used to determine whether or
not urine contains the
hormone and if a pregnancy
exists.
After a placenta has become established, it will begin producing and releasing
progesterone itself. The placenta and the umbilical cord permit nutrients and waste
materials to pass back and forth between the mother and embryo(s). Exchanges
between mother and embryo take place at the placenta. There is no direct joining of
the two circulatory systems. Rather, substances diffuse back and forth through the
lymph between the two systems.
The graph on the right shows the hormone
levels as they are during a pregnancy.
Compare this graph back to the graph on
the top of page 72 that shows the hormone
levels in the regular monthly cycle. (You
should be familiar with this graph as well.)
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Occasionally, in the blastula or early gastrula stages, embryonic
cells could divide roughly in half and then go on developing as
separate individuals. This action gives rise to identical twins or,
if divisions are not complete, Siamese twins (joined together).
Human embryological development is generally studied on the basis of trimesters, or
three month developmental periods.
First Trimester: During the first trimester, most human embryos will be forming all
their limbs and other organs. For this reason, the mother's health, diet and whatever
enters her body in the early stages of pregnancy can have significant final results. At
the end of the first trimester, all the major body organs, systems and external body
features will have been formed. Usually weighing about one gram, a 2 to 3 cm
embryo begins to be called a fetus before the first trimester is over.
Second Trimester: Remaining growth in the uterus largely consists of adding more
cells to the tissues and organs which were developed in the embryonic stages. Leg,
arm and general movements of the fetus begin about the fourth month of pregnancy
and increase in frequency and force to about the sixth month. At the end of this
second trimester, the baby is "almost" ready to be born, with all systems complete.
Third Trimester: Most vital systems are still frequently too weak to function
independently if a premature birth took place at the end of the sixth month.
Movements tend to decrease after the second trimester as the fetus' increased size
gives it less room to maneuver in a more confined space. Large weight gains take
place in the last three months, slowing only in the latter stages of the ninth month.
The last number of weeks of a pregnancy have a fetus assuming a lower position in
the uterus with the head normally positioned at the lower end, near the cervix.
During this time the placenta will usually have started to degenerate as well.
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Fetal Health
The development of a fetus is dependant on the health of the mother. As well as
providing beneficial substances, such as vitamins and minerals, to the baby, a
mother can transfer harmful substances as well. These harmful substances can
greatly affect the embryo’s development. Whatever the mother ingests or inhales can
end up in her blood, which in turn can be passed to the fetus. The developing fetus
is at greatest risk during the first 9 weeks of development when the organs are
developing.
INSERT DIAGRAM HERE ILLUSTRATING THE EXCHANGE
Placenta and Substance Exchange
Substances harmful to the fetus, causing structural abnormalities, are termed
teratogens. Teratogens can include:




cigarette smoke – may lead to underweight babies
alcohol – can affect the fetus’s brain and nervous system. Alcohol consumption
can lead to babies which have Fetal Alcohol Syndrome, FAS. Children with FAS
can have smaller weight, height, and head size, malformations of the head and
face, and varying degrees of mental retardation
some prescription and over the counter drugs - thalidominde was a drug in the
1950’s that was used to treat morning sickness. This resulted in many babies
being born with missing or deformed limbs.
Radiation – such as X-rays. X-rays are generally not performed on pregnant
women, unless the benefit outweighs the risks
Proper nutrition along with avoidance of teratogens is essential for a pregnant women
in order to signficantly decrease the chance of having a child with a birth defect.
However, some birth defects are due to heredity or other unknown, unavoidable
factors.
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Lesson 11
The Birth Process
The actual development period or gestation in placentals varies considerably. In
rodents, the time from fertilization to birth could be only three to four weeks.
Members of the dog family have gestation times of approximately two months. Many
other mammals, including humans, have times averaging about 9 months. One of
the longer gestation periods is that of an elephant, which could be up to 22 months.
The factor(s) which actually start the birth process are still not clear. Whatever the
triggering agent(s), hormone levels seem to be part of the action. During pregnancy,
estrogen and progesterone hormones are in balance with one another. The first has a
tendency to cause contractions of the smooth, uterine muscle. The second,
progesterone, prevents contractions. Just before birth, levels of progesterone drop.
About this time, the pituitary begins increasing its output of another hormone called
oxytocin. The concentrations of oxytocin and estrogen (whose levels now become
higher that those of progesterone) finally cause uterine contractions to begin. In
certain situations, doctors will sometimes inject oxytocin to induce contractions and
labor.
The first part of the birth process generally takes the longest. While uterine
contractions increase in frequency, muscles of the cervix and vagina relax. This
dilation or expansion permits easier passage of the young animal or human baby.
Dilation could last from approximately an hour to over fifteen hours. Females who
gave birth previously generally have shorter dilation times. The end of the dilation
period comes with the passage of a mucus plug which had been in the cervical
opening during pregnancy. The amnion membrane will have been ruptured by
muscle contractions just before this so that amniotic fluids will be expelled as well.
This "water breaking" is followed by the second part of birth, or true labor, in which
the fetus is pushed out. This could last from several minutes to over an hour.
True labor sees another example of a feedback effect on the body. Usually, feedbacks
have negative or inhibitory effects on the initial actions. During this time, there is a
positive feedback, where a particular action results in an effect which further
intensifies the action and effect. As the baby's head presses against the smooth
(involuntary) muscles, the pressure actually causes those muscles to contract even
more frequently and with more strength. This effect goes on until the baby is finally
pushed out. With a decrease in the pressure on the smooth muscles, the
contractions will begin to slow down, although still continuing for a time afterward.
Emergence of a young animal or baby could cause the umbilical cord to tear from the
placenta still in the uterus. (In humans it is usually cut just after emergence of the
baby.) The torn or cut umbilical cord quickly seals so that there is no loss of blood.
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After developing in a warm, watery world, the newborn will be stimulated to begin
breathing by the licking or touching actions of the mother and increasing CO2 levels
in its own blood. The final stage of birth is the expulsion of the placenta and any
remaining fetal matter. This is accomplished by the contractions of the smooth
muscles which still continue, but at diminishing rates. The contractions also help to
seal off broken blood vessels in the uterine wall.
In a week to several weeks after birth, the mother's uterus returns to normal size.
Any tissue damage to the vaginal walls which occurred during birth heal. Hormone
levels and concentrations return to conditions as they were prior to pregnancy –
unless a mother is actively feeding young with her own milk. In such a situation,
there could be higher levels of progesterone in the mother's body. This could delay
the return of estrus or menstrual cycles until weaning of the young occurs.
Offspring Survival
In many populations, mortality rates are frequently highest among the very young –
during embryonic development and shortly after birth. Therefore, to consider the
success of reproduction, one should also include a time period shortly after hatching
or birth.
Different species have different adaptations to increase survival chances of their
young. For many, parental involvement is very limited. Many fish, amphibians and
reptiles deposit their eggs and then leave them. For such species, the poor survival
rates among the young are compensated for by very high egg numbers. Eggs are laid
by individuals in numbers ranging from thousands into the millions. Predation upon
the young could still leave some unharmed. Large numbers could also be an
advantage in providing early warning signals to a group. As well, sometimes the
scattering action of many individuals can be confusing to predators.
Internal fertilization and internal development of embryos offers greater protection for
the young. Limited internal space reduces the numbers of offspring produced, but
the better protection increases their survival chances. Mothers, and often both
parents, offer varying amounts of care and protection to the young. Parental
involvement frequently increases as number of offspring decreases. One or both
parents could build burrows, nests or other shelters for the young. Protection is
provided by the parents acting as decoys to draw predators away or actually
attacking them.
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Lesson 11
Offspring themselves can show different survival techniques. Young of many species
become quiet and almost completely motionless when there is danger nearby. Some
have the added protection of having very little body scent initially, so as not to attract
possible predators. Body markings could provide effective camouflage with the
environment. Some young can "bluff" very effectively with threatening postures,
hisses, loud squeals or snapping and biting actions. Fluffing feathers or holding
wings outward, or having body fur rise, have the effect of making some young
organisms seem much bigger than what they are.
In species with longer gestation periods, offspring usually show a dependency on the
parent(s) for longer periods of time. The parent(s) not only provide food and
protection, but may teach their youngsters survival skills. Some offspring remain
closely associated with their parents until they become sexually mature.
Other Aspects Related to Human Reproduction
Monitoring Human Reproduction and Embryo Development
The births of children with various structural defects or defective body functions,
along with their ensuing consequences, have long been a concern to many people.
Such concerns have led to efforts to find ways of determining the characteristics of
young before they are even born. Currently, there are a number of ways of carrying
out some examinations.
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Lesson 11
Ultrasound Imaging



Utilizes sound waves which
pass through a baby's body
to produce an image on a
screen.
Images or outlines of
various body features may
indicate possible structural
abnormalities.
Routinely performed at 16
weeks gestation.
Amniocentesis


Drawbacks

Some individuals express
concern with this
procedure, due to the lack
of information as to what
effect(s) sound waves may
have on an embryo.


Involves the insertion of a
syringe through the abdominal
wall of a mother and into the
fetal sac.
A quantity of amniotic fluid,
containing some of the
embryo's cells, is removed for
analysis. Some of these
embryo cells are cultured in a
special medium until a
karyotype can be prepared. A
karyotype shows the number
of chromosomes present, their
sizes and shapes. An
examination of the
chromosomes may determine
any possible abnormality
about them and may perhaps
indicate some consequence of
this. An examination of the
amniotic fluid itself is also
carried out to see if it contains
any unusual proteins.
Tests on the cells and fluid are
able to identify certain specific
disorders which may be
present in a developing
embryo or fetus.
Generally performed only on
pregnant females over the age
of 35 (risk of genetic
abnormailites increases after
age 35)
Chorionic villus biopsy



Involves the insertion of a
thin tube through the
mother's vagina and into the
uterus.
A quantity of cells can be
removed from the chorionic
membrane surrounding the
embryo. As the chorion
originates from the same
cells as the embryo, it has
the same genetic component.
A karyotype can then be
examined
Can be performed at about
two months into the
pregnancy.
Drawbacks

The risk of a possible
spontaneous abortion is
slightly higher with this
procedure than with
amniocentisis
Drawbacks:

requires care, to avoid any
possible injury to the fetus
and to avoid a possible
spontaneous abortion.
 not usually performed before
the fourth or fifth month of a
pregnancy due to the small
size of the amniotic sac and
small quantity of amniotic
fluid prior to that.
Since testing is not done until
later during gestation, discovery of
an abnormality limits the kinds of
actions which may be taken to
deal with it.
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The results of tests, such as these and others, are generally positive and confirm
normal pregnancies and developments. There are some, however, that require
special medical or genetic counselling and possibly specific actions. Embryonic
disorders could require such actions as blood transfusions to embryos, having
mothers put on special diets or drugs, or various other procedures. Genetic
counselling is frequently sought out by individuals who have some knowledge of
certain disorders, their genetic backgrounds or their family histories. People may
want extra information with respect to such things as: how certain disorders
progress; probabilities of their children inheriting or experiencing certain conditions;
genetic or diagnostic tests which could be carried out; interpretation of certain
results; and, possibly, advice or recommendations on particular courses of action.
Birth Control
For a variety of reasons, individuals may choose not to have children or to delay
having children until particular times. As numerous as the reasons may be, so may
the ways of preventing reproduction.
Permanent surgical sterilizations are used to prevent the movements of gametes to
areas where fertilization could occur by cutting off access within the duct networks.
In females, abdominal incisions can be made so that access to the oviducts permits
them to be tied off somewhere along their lengths. This tubal ligation prevents the
movement of eggs from ovaries to the uterus.
INSERT DIAGRAM OF TUBAL LIGATION HERE
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Lesson 11
In a vasectomy, a male can have small incisions made in the sides of the scrotum or
sac holding the testes. The vas deferens along the sides are exposed, tied off in two
places and then surgically cut between the two tie-offs. This prevents sperm from
moving out of the testes.
INSERT DIAGRAM HERE
Vasectomies and tubal ligations are reversible, but the reversal may or may not be
successful.
In other male animals, the practice of castration involves the removal of the entire
testes. Doing this means that there will be no sperm and also no more hormone
production from certain cells located in the testes.
The following table summarizes some common methods of birth control.
Methods of Birth Control
Insert table here if desired that compares methods such as abstinence,
vasectomy, birth control pill, tubal ligation, needle injections or implants.
Sterility
On the opposite end of the spectrum from birth control, is the situation where some
individuals or couples are unable to have children.
Causes of Female Sterility
Causes of Male Sterility

blocked fallopian tubes


failure to ovulate, due to
hormonal imbalance
Obstruction in the vas
deferens or epidydymus

Low sperm count

endometriosis

Abnormal sperm

damaged eggs
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Lesson 11
There are a number of options available to overcome sterility.




Often surgical techniques can be used to remove blockages
Hormonal problems can also be corrected. Low progesterone could prevent an egg
from embedding in the uterine wall. Taking extra progesterone around the time of
ovulation would correct this problem. Sometimes, follicle cells have low
sensitivities to the hormones FSH and LH. Fertility drugs, which mimic the
hormone put out by the hypothalmus to stimulate FSH and LH production, can be
taken. Much greater levels of these two hormones could then initiate follicle
development. Often, these fertility drugs are too effective and there are multiple
follicle developments and multiple births.
Low sperm counts could be enhanced by combining a number of ejaculations in a
laboratory to prepare one sample with an about normal sperm
Other technologies are summarized below.
Insert table here that compares AI, IVF, IVM, superovulation, surrogate or
cryopreservation if desired.
Some of these procedures have moral or "ethical" issues associated with them. A
fertilized egg could be implanted into a different female than the one from which it
was removed. It could be that a father's identity may not even be known, with donor
sperm coming from sperm banks. Individuals could be born without ever knowing
the identity of either biological parent. It is also possible to determine the sex of the
embryo and to manipulate or to modify it genetically, before implantation.
Sexual Diseases (STD’S)
Internal fertilization and contact between sexual partners bring with them the
possibilities of transferring viruses, bacteria or other parasites between partners.
Syphilis (bacterium), gonorrhea (bacterium), chlamydia (bacterium), herpes (viral)
and AIDS (viral), are just some diseases. Sexually transmitted, or venereal, diseases
increase in frequency where there are multiple partners. Prevention is the most
effective way of dealing with such diseases and reducing the number of partners is
one preventative measure. Aside from abstinence, the condom is the most effective
method for prevention of sexually transmitted diseases. Most diseases can be cured if
diagnosed early; however, immunity usually doesn't last. Initial infections may not
be noticed or may seem too minor to be of any consequence. Embarrassment can
also be a factor in not seeking medical attention. Lack of medical attention and help
can have undesirable effects in a number of ways. Without treatment, other partners
and even unborn babies are at risk of being infected. Development of a specific
disease can possibly lead to sterility, more serious internal damage and even death.
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Lesson 11
Summary
Vertebrate reproduction is accomplished through the sexual process, where different
gametes fuse together. The sexual process restores the normal diploid chromosome
number of the species in a new zygote. Fertilization can be either external or
internal. Water for sperm transfer is especially important in external fertilization, but
body fluids are also necessary for transporting sperm and eggs when internal
fertilization is carried out.
Embryonic development can take place within eggs. Some eggs are fertilized
externally and develop in an aquatic or moist environment so that they will not dry
out. Reptiles and birds have amniotic eggs, with outer protective shells and inner
membranes enclosing watery environments. The membranes are important in the
movements of nutrients and wastes in and out of embryos. Some ovoviviparous
species retain the eggs internally until they hatch. Young are then born to begin
independent lives. Yolk material is fairly prominent and is important in embryo
developments within eggs. Marsupial embryos have only a limited yolk supply.
Young are born only after brief internal developments. A precarious journey must be
made by the underdeveloped young to a marsupium or pouch where growth can be
completed. In terms of security, vertebrate placentals provide the best conditions for
embryo and fetal development. Extraembryonic membranes, modified into placenta
and umbilical cord, serve as the lifeline between mother and young until birth. Once
viviparous development is complete, youngsters are born into conditions where their
parents often provide some degree of support for a time afterwards. Species with
longer gestation times and where young are slower to mature, generally show longer
parent-offspring relationships.
Although differences do exist in reproductive methods, there are basic similarities
among all species. Small parts of parent organisms develop into new individuals.
These small parts or initial beginnings contain all the necessary material and genetic
information that is needed to maintain species survival. In this way, a continuity of
life is maintained generation after generation, as long as reproduction remains
successful.
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