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BIOL 129 Animal Development Procedure
This lab is a comparative study of early development in four organisms: (1) the
sea star; (2) the frog; and (3) the chick; and (4) the human.
Development in an Echinoderm: The Sea Star
Materials
compound microscope
prepared slide of whole sea star embryos in different stages of development
Introduction
The sea star (starfish) is classified in the phylum Echinodermata, the invertebrate group
that is phylogenetically closer to chordates than any other. Male and female sea stars
release large numbers of gametes into the sea, and fertilization is external. Early
development leads to a larval stage that is free-swimming and free-feeding. In this
exercise, you will observe slides that contain whole embryos in various stages of
development. You will identify each developmental stage and determine the type of egg
and cleavage pattern of the echinoderm.
Procedure
1. View the prepared slide of sea star embryos using low and intermediate powers on
the compound microscope.
Use only 10X and 40X lenses for the sea star slides. Using the high power
objective to view this slide will destroy the slides!
2. Find examples of the following stages of development. When you find a good
example of each of the stages described, make a careful drawing of that stage in the
appropriate square in Figure 5. Draw only one gastrula. Also answer the questions
below in the spaces provided.
Egg
By the time sea star eggs leave the body of the female, meiosis I and II are completed.
The nucleus, called the germinal vesicle, is conspicuous because the nuclear envelope
is intact. A nucleolus is usually distinct. The plasma membrane surrounding the egg
cytoplasm closely adheres to a thin external membrane known as the vitelline layer. The
vitelline layer contains species-specific sperm receptors.
The fertilized egg, or zygote, has no visible nuclear envelope, giving this cell a uniform
appearance.
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Early Cleavage
Cleavage begins with the zygote and converts this single cell into a multicellular
embryo. Find two-, four-, and eight-cell stages. Is the entire zygote involved in
early cleavage?
What is happening to the size of the cells as cleavage takes place and cell
numbers increase?
Blastula
As cleavage continues, a cavity, the blastocoel, forms in the center of the cell cluster.
The end product of cleavage will be a hollow ball of cells, the bIastula. Locate and study
a blastula. How does the size of individual cells compare with the size of the
fertilized egg?
How does the overall size of the blastula compare with that of the fertilized egg?
Early Gastrulation
Gastrulation converts the blastula into the gastrula, an embryo composed of three
primary germ layers. The early gastrula can be recognized by a small bubble of cells
protruding into the blastocoel. These cells push into the blastocoel through a region on
the embryo surface called the blastopore. As cells continue to invaginate, or move
inward, a tube called the archenteron forms. The archenteron eventually becomes the
adult gut. Which embryonic germ layer lines the archenteron?
Middle Gastrulation
The archenteron continues to grow across the blastocoeL It takes on a bulb-like
appearance as the advancing portion swells.
Late Gastrulation
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Cells at the leading edge of the advancing archenteron extend pseudopodia that
attach to a specific region across the blastocoel. These cells continue to pull the
archenteron across the blastocoel. As the tip of the archenteron approaches the
opposite wall of the embryo, it bends to one side and fuses with surface cells. The site
of fusion will eventually become the mouth of the embryo. What will be formed from
the blastopore at the opposite end of the archenteron?
What is the germ layer of cells on the surface of the embryo called?
The amoeboid cells that attach the archenteron to the embryo wall are called
mesenchyme cells. These cells later detach from the archenteron, proliferate, and form
a layer of cells lining the old blastocoel, now divided by the archenteron. This layer of
cells will become the mesodermal germ layer
Figure 5. Early stages of development in the sea star. Draw an example of each stage
in the spaces below.
Egg
Cleavage (4 or 8 cell stage)
Blastula
Gastrula
Development in an Amphibian
Materials
compound microscope
prepared slide of frog embryos at the neurula stage of development
Introduction
Amphibians are vertebrates that lay jelly-coated eggs in water or in moist areas on land.
Common examples include frogs and salamanders. For most species, fertilization is
external, with the male depositing sperm over the eggs after the female releases them.
Internal fertilization takes place in some amphibians, however, in which cases the young
are born in advanced developmental stages. Early development is similar in all species.
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After fertilization, the zygote begins cleavage followed by gastrulation, neurulation, and
organogenesis.
Procedure
1. Look at a slide showing frog neurulation. Refer to Figure 4.
In this strictly chordate process, certain ectodermal cells flatten into an elongated neural
plate extending from the dorsal edge of the blastopore to the anterior end of the
embryo. The center of the plate sinks, forming a neural groove. The edges of the plate
become elevated to form neural folds, which approach each other, touch, and
eventually fuse, forming the hollow neural tube. The anterior end of the tube develops
into the brain, while the posterior end develops into the nerve (spinal) cord.
Results
1. Draw two representative sections of a frog neurula; one showing the neural folds and
the other showing the neural tube.
Figure 6. Frog neurula drawings
Neural folds
Neural tube
Development in a Bird: The Chicken
Materials
compound microscope
unincubated egg (demonstration
prepared slide of 16-20 hour
prepared slide of 24-hour chick
prepared slide of 33-hour
prepared slide of 48-hour chick
Introduction
Immature eggs, or oocytes, develop within follicles in the single ovary of the adult
female bird. (Two ovaries begin to develop in birds, but the second ovary degenerates.)
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In the sexually mature bird, hormonal stimulation brings about ovulation, the release of
oocytes into a single oviduct. An oocyte consists of active cytoplasm called the
blastodisc, or germinal disc, floating on a huge amount of food reserve, the yolk,
surrounded by a plasma membrane. At the time of ovulation, chromosomes in the large
oocyte nucleus have just completed the first maturation division of meiosis (meiosis I).
At ovulation in chickens, the oocyte nucleus measures approximately 0.5 mm and the
oocyte measures approximately 35mm in diameter.
Fertilization is internal in birds, if sperm are present in the oviduct at ovulation, they will
penetrate each oocyte (one per oocyte), stimulating the completion of meiosis in the
oocyte nucleus. The sperm nucleus and the now mature egg nucleus fuse, producing
the zygote nucleus, which begins to divide by mitosis followed by cytoplasmic cleavage.
As this developing embryo continues its passage down the oviduct, albumin, shell
membranes, and finally a calcareous shell are deposited on its surface. In chickens,
passage down the oviduct takes about 25 hours. This means that a freshly laid chicken
egg, if it has been fertilized has completed about 25 hours of development. The cleaved
blastodisc is now called the blastoderm, or blastula. Development continues in the
blastoderm giving rise to all parts of the embryo, with yolk containing carbohydrates,
proteins, lipids, and vitamins serving as the food reserves.
In this exercise you will observe an unincubated egg and incubated eggs in several
stages of development. As you study the embryos, identify developing structures and
compare bird development with that of the sea star and frog.
Procedure
1. Refer to Figure 7 and observe the unincubated chicken egg on demonstration.
Most eggs sold for human consumption are purchased from commercial egg farms
where hens are not allowed contact with roosters.
a. Observe the broken calcareous shell with outer and inner shell membranes just
inside. The shell and membranes are porous, allowing air to pass through to the embryo
inside. You have probably noticed an air chamber at one end of a hard-boiled egg,
between the two membranes.
b. Observe the watery proteinaceous egg albumin (egg white) and the yellow yolk. The
layers of albumin closest to the yolk are more viscous and stringy than the outer
albumin. As the yolky egg passes down the oviduct, it rotates, twisting the stringy
albumin into two whitish strands on either end of the yolk. Called chalaza, these strands
suspend the yolk in the albumin.
c. Locate the cytoplasmic island, a small whitish disc lying on top of the yolk. This is
larger in a fertilized egg because of the development that has taken place. Remember
that this island is called the blastodisc before cleavage begins and the blastoderm after
cleavage has begun. Cleavage is restricted to the blastoderm; the yolk does not divide.
As cleavage takes place, the blastoderm, now a mass of cells, becomes elevated above
the yolk. Subsequent horizontal cleavages create three or four cell layers in the
blastoderm, and a space, the blastocoel, forms within these layers.
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Figure 7. The unincubated chicken egg. The chalaza suspends the yolk in the albumin.
2. Study the prepared slide of the 16-hour egg (the gastrula). Refer to Figure 8a, a
surface view of the embryo.
Use only 4X and 10X objective lenses when viewing the chick slides.
The 40X and 100X objective lenses will break the slide
a. Using low and then intermediate powers on the compound microscope, view a
prepared slide of the whole chick embryo after about 16 to 20 hours of incubation. At
this stage, cells in the blastoderm have separated into an upper layer forming ectoderm
and an inner layer forming endoderm. These layers are visible only in sections of the
embryo.
b, Locate a dark, longitudinal thickening, the primitive streak. Surface cells migrate
toward the primitive streak and then turn under, through the primitive streak. By 18
hours, they have spread out and initiated the formation of mesodermal tissues (Figure
8b). The notochord is one of the first structures to develop in the mesodermal layer.
c. Draw the 16-20 hour embryo below.
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Fig. 8. Chick gastrulation. (a). Surface view of chick blastoderm after 16 hours of
incubation, the gastrula stage. The primitive streak is visible. (b). Cross section of
blastoderm after 16 hours of incubation. Cells turn in at the primitive streak, initiating
the formation of mesodermal tissues.
3. Study the prepared slide of the 24-hour chick (neurulation). Refer to Figure 9.
a. Use the low and then intermediate powers on the compound microscope to view the
prepared slide of a chick after 24 hours of incubation. At this stage, the neural tube is
forming anterior to the primitive streak in a process similar to neurulation in fish.
b. Look for a longitudinal ectodermal neural plate with elevated edges called neural
folds and a depressed center called the neural groove. The margins of the neural folds,
which appear as a pair of dark longitudinal bands, become elevated and approach each
other until they touch and eventually fuse. This fusion of the folds forms the neural tube.
The anterior end of the neural tube becomes the brain; the posterior end becomes the
spinal, or nerve, cord.
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c. Label the following on Figure 9: neural folds, neural groove, developing brain,
spinal cord region of the neural tube, and primitive streak.
Figure 9. The chick after 24 hours of incubation (neurulation. Edges of the ectodermal
neural plate elevate, forming neural folds. The depressed center is the neural groove.
The neural folds eventually fuse, forming the neural tube.
4. Study the prepared slide of the 33-hour chick (post-neurulation). Refer to Figure 10.
a. View the prepared slide of a chick after 33 hours of incubation. At this stage, the
heart is forming as a S-shaped tube with the following parts: conotruncus (ct), ventricle
(v), atrium (a), and sinus venosus (sv) are evident.
b. Note the continued development of the brain and neural tube. Additional somites are
also evident along the sides of the neural tube
c. Label the following on Figure 10: brain, neural tube, somites
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Figure 10. The chick after 33 hours of incubation. The conotruncus (ct), ventricle (v),
atrium (a), and sinus venosus (sv) are evident within the S-shaped heart.
5. Study the 48-hour chick (early organogenesis), Refer to Figure 11.
a. Identify structures in the circulatory system. In the living embryo the heart is already
beating at this stage, pumping blood through the vitelline blood vessels, which emerge
from the embryo and carry food materials from the yolk mass to the embryo. The atrium
lies behind the ventricle, which is a larger, U-shaped chamber.
b. Identify structures in the nervous system. The anterior part of the neural tube has
formed the brain. Eyes are already partially formed. Follow the tube posteriorly to the
spinal cord.
c. Observe somites, blocks of tissue lying on either side of the spinal cord. Somites
develop into body musculature and several other mesodermal organs.
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d. Label Figure 11.
Figure 11. the chick after 48 hours of incubation. Identify the heart (atrium and
ventricle), vitelline blood vessels, the brain, an eye, the spinal cord, and somites.
Development in a Human
The human embryo, and other mammals, also contains the same four extra-embryonic
membranes as the chick embryo, but with often different functions. The chorion helps
form the placenta. The yolk sac forms the first blood cells. The amnion still forms a fluidfilled sac surrounding the embryo. The allantois helps form the umbilical cord.
Examine the models illustrating human gastrulation. Human gastrulation follows much
the same scenario as that of the chicken. However, before gastrulation a cavity forms
between the inner cell mass and the trophoblast. This is the cavity of the amnion. Also,
cells from the inner cell mass grow downward, along the inner surface of the
trophoblast, fuse, and form the yolk sac. Between the amniotic and yolk cavities is the
embryonic disk. The embryonic disk in mammals is the equivalent of the blastoderm in
the chicken.
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Implantation of the embryo in the uterus occurs at the same time as the formation of
the amnion and yolk sac, about eight days after fertilization of the ovum in the upper
oviduct. The outer layer of the trophoblast erodes away the maternal tissues so that the
blastocyst can sink into the wall of the uterus. The inner layer of the trophoblast forms
the chorion.
Observe the models of the early stages of the human development.
Go to the Visible Embryo web site - http://www.visembryo.com/baby/index.html
According to the web site “The Visible Embryo is a visual guide through fetal
development from fertilization through pregnancy to birth. As the most profound
physiologic changes occur in the "first trimester" of pregnancy, these Carnegie stages
are given prominence on the birth spiral.” The stages are important landmarks in fetal
development.
Draw the Carnegie stages 2, 4, 5, 7, 10, 12.
Q. What day(s) post fertilization do the stages represent?
Q. What does the embryo look like at each stage?
Q. What systems, organs, are beginning to form at each stage?