CHAPTER 53: ANIMAL DEVELOPMENT
WHERE DOES IT ALL FIT IN?
Chapter 53 builds on the foundations of Chapter 32 and provides detailed information about animal
form and function Students should be encouraged to recall the principles of eukaryotic cell structure
and evolution associated with the particular features of animal cells. Multicellularity should also be
reviewed. The information in chapter 53 does not stand alone and fits in with the all of the chapters
on animals. Students should know that animals and other organisms are interrelated and originated
from a common ancestor of all living creatures on Earth
SYNOPSIS
The process of fertilization, the uniting of male and female gametes, is divided into three phases:
penetration, activation, and fusion. In mammals, once the first event has occurred, no additional
sperm are able to fertilize the egg. Eggs of some animals can be activated by physical stimulation
and thus be induced to develop parthenogenetically. After fertilization, the zygote undergoes
several mitotic divisions (cleavage) producing a greater number of smaller cells.
The patterns of cell cleavage in vertebrates are influenced by the amount and location of yolk in
the developing egg. Primitive aquatic vertebrates exhibit a symmetrical blastula with evenly
sized cells. Advanced fish and amphibians develop an asymmetrical blastula with small cells at
one pole and large cells at the other. The blastula of reptiles and birds is a small disk located on
top of the yolk. Mammal embryos develop internally, the blastula resembles that of a reptile
although there is little yolk present. The structure of the blastula determines future
developmental events as cells in different positions contain different portions of the original egg
cytoplasm, signal substances that affect development. Each cell is also in contact with different
sets of neighbor cells, influencing development through positional information. At this stage,
each cell has already been given its own unique developmental fate.
The formation of primary ectoderm, endoderm, and mesoderm is a result of an event called
gastrulation. The patterns of gastrulation vary among the different kinds of vertebrates. The
gastrula forms by simple invagination (primitive chordates), movement of layers of cells over the
yolk (aquatic vertebrates), or by cell differentiation without movement (reptiles, birds, and
mammals). In mammals, the mesoderm arises from the cells of the upper layer, the ectoderm.
The ectoderm eventually becomes the epidermis and neural tissue. The mesoderm forms
connective, muscular, and vascular tissue. The endoderm becomes the lining of the gut and its
derivative organs.
Neurulation occurs only in chordates and results in the formation of the notochord and the
hollow dorsal nerve cord. The formation of the neural crest occurs only in vertebrates, resulting
in the development of the gill chambers, the nervous system, the skull, and various sensory
organs. In determination, the developmental fate of a cell can be predicted and occurs very early
in development. A “committed” cell’s developmental fate cannot be altered and occurs later.
The evolution of neural crest was critical to the development of vertebrate predatory
characteristics. These include greater physical activity, improved sensory detection of prey,
improved spatial orientation, and quicker sensory responses. Patterns of development are built
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upon other patterns that occurred in earlier forms. For example, mammal development elaborates
on reptile development which elaborates on amphibian development. This evolution of
development gave rise to the phrase “ontogeny recapitulates phylogeny,” true with the
consideration that embryonic stages reflect their embryonic rather than adult ancestry.
Terrestrial vertebrates, including humans, evolved membranes derived from embryonic tissue,
but not contained within the embryo—the extraembryonic membranes that include the amnion,
chorion, yolk sac, and allantois. The amnion surrounds the embryo bathed in seawaterlike
amniotic fluid. In birds and reptiles, the chorion lies immediately inside the egg shell. In
mammals, the part of the chorion that is in contact with the endometrium becomes the fetal
placenta. Development in humans requires a nine-month commitment by the mother. Pregnancy
is frequently undetected in the first month even though organogenesis in the embryo is complete
at this time. Morphogenesis occurs in the second month, and by the end of the third month the
limbs of the fetus are fully formed and show movement. Significant growth occurs during the
second and third trimesters; the weight of the fetus doubles several times. In some mammals,
birth is initiated by changes in fetal hormone levels. In humans, an increasing amount of
placental estradiol causes the uterus to release prostaglandins. Sensory feedback stimulates the
release of oxytocin, also associated with uterine contractions. Lactation and the milk-ejection
reflex require different hormones. Rapid growth in the baby continues after birth, especially in
terms of neural and sensory development.
LEARNING OUTCOMES
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Describe the events of each of the three stages of fertilization in advanced vertebrates.
Compare the cleavage patterns found in primitive chordates, fish/amphibians, reptiles/birds, and
mammals, include the amount of yolk found in the egg and the type of blastula that is formed.
Describe the appearance of the blastula and indicate how gastrulation proceeds in primitive
chordates, aquatic vertebrates, and reptiles/birds/mammals.
Understand how chordate development is different from the less advanced vertebrates, especially
in terms of neurulation and formation of mesodermal tissue.
Understand the importance of neural crest formation in vertebrates and indicate the structures
associated with this stage.
Explain Haeckel’s “biogenic law,” ontogeny recapitulates phylogeny.
Understand the importance of extraembryonic membranes in terrestrial vertebrate development.
Describe the characteristic events of each month of the first trimester of human pregnancy.
Compare the initiation of birth in humans to that of most other mammals.
Describe the characteristic events of the second and third trimesters of human pregnancy.
Explain how early postnatal growth differs in human and nonhuman vertebrates and understand
the factors that affect brain growth and development.
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COMMON STUDENT MISCONCEPTIONS
There is ample evidence in the educational literature that student misconceptions of information
will inhibit the learning of concepts related to the misinformation. The following concepts
covered in Chapter 53 are commonly the subject of student misconceptions. This information on
“bioliteracy” was collected from faculty and the science education literature.
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Students are unaware of the similarity of embryological development of animals
Students are unaware of the role of hormones and cell signals in embryological
development
Students believe that the environment plays little role in embryological development
Students believe that animals exclusively reproduce sexually
Students are unaware of that plancental animals produce eggs
Students believe that all animals have internal fertilization
Students are confused by the distinction between plancenta and ovoviviparity
Students have a misunderstanding of the anatomical features of hemaphrodites
Students believe that all hermaphrodites can fertilize themselves
Students believe that estrogen is only hormone involved in the menstrual cycle
Students think that all animals evolved at about the same time
Students believe that most animals are vertebrates
Students do not equate humans with being animals
Students believe that all animals have identical organ system structures
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
Stress that only chordates undergo neurulation and only vertebrates form the neural crest. This
shows evolution at work, improving a basic pattern by the addition of successive small steps.
Discuss the need for brain growth after birth, especially in hominids. The size of the head must
be small to pass through the birth canal. External stimuli are also required to form nerve tracts
and neuronal associations. The latter is not as necessary in the lower vertebrates and more
primitive mammals as most early activity is innate.
HIGHER LEVEL ASSESSMENT
Higher level assessment measures a student’s ability to use terms and concepts learned from the
lecture and the textbook. A complete understanding of biology content provides students with the
tools to synthesize new hypotheses and knowledge using the facts they have learned. The
following table provides examples of assessing a student’s ability to apply, analyze, synthesize,
and evaluate information from Chapter 53.
Application
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Have students explain how diploidy is ensured during sexual reproduction
each generation.
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Have students explain the value of extraembryonic membranes in the
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survival of land animals.
Analysis
Synthesis
Evaluation
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Ask students how variations in yolk content in an egg affect gastrulation.
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Have students explain any value to the evolution of extraembryonic
membranes in aquatic animals.
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Have student describe how mutations to genes associated with
neuralization would affect an organism’s development.
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Ask students to explain the relations between hox genes and
somitogenesis.
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Ask students to design an experiment to test relationship between
environmental temperature and egg development in amphibians.
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Have students come up with an application of the knowledge that human
blastomeres are not committed to a particular path.
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Ask the students come up with strategy to test when mammalian
embryonic cells become committed to forming one lineage of cell types.
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Ask students evaluate the use of fetal stem cells for regenerating damaged
human tissues.
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Ask students to access the accuracy of using amphibian eggs as a model
for human birth defects.
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Ask students investigate the pros and cons contraceptive strategies that
prevent the blastula from attaching to the lining of the uterus.
VISUAL RESOURCES
Bring in embryology models or use overhead diagrams. Animations and movies about
development in different organism are useful for showing concepts that cannot demonstrated in
easily during lecture.
IN-CLASS CONCEPTUAL DEMONSTRATIONS
A. Fetal Development Timelines
Introduction
This demonstration uses instructor-controlled animations to help students understand the
stages of human embryology.
Materials
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Computer with Media Player and Internet access
LCD hooked up to computer
Web browser linked to Boston University histology website at
http://www.umm.edu/aniplayer/index.html
Procedure & Inquiry
1. Tell the class they will be viewing animations about embryological development.
2. Load up the University of Maryland Medical School website and click on the
Reproductive System bar
3. Select Fetal Development Interactive Tool
4. Go through the two animations
5. Have students describe the stages of development that they viewed
6. Then go through the formation twins movie
7. Have students describe the events they viewed
USEFUL INTERNET RESOURCES
1. The University of Kansas Medical Center has a valuable histology website called
HistoWeb. It has many clear microscopic images of female and male reproductive tract
photomicrographs. The site is available at
http://www.kumc.edu/instruction/medicine/anatomy/histoweb/
2. Valuable comparative embryology information is hosted by the WGBH Educational
Foundation and Clear Blue Sky Productions. It has classical and contemporary
information and images that can be shared with the class. The website can be found at
http://www.pbs.org/wgbh/evolution/library/04/2/pdf/l_042_03.pdf.
3. The University of New South Wales in Australia has a useful embryology research
website. It has many images and a wealth of information for classroom use. The website
can be found at http://embryology.med.unsw.edu.au/OtherEmb/EmbHome.htm.
4. Case studies are an effective tool for stimulating interest in a lesson on animals. The
University of Buffalo has a case study called “Selecting the Perfect Baby: The Ethics of
"Embryo Design” which has students investigating the ethical issues associated with
genetic manipulation of embryos. The case study can be found at
http://ublib.buffalo.edu/libraries/projects/cases/genetics/genetic_selection.pdf.
LABORATORY IDEAS
A. Embryo Issues Virtual Lab
This activity has students investigate the effects of teratogens on human development.
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a. Explain that they will be using a virtual human embryology website to answer questions
about toxicological effects of pollutants on human development.
b. Provide the students with the following:
a. Computers with Internet Access
b. Web browser bookmarked to Visembryo Website at:
i. http://www.visembryo.com/baby/index.html
c. Tell students to explore the Visembryo website and familiarize themselves with the
stages of embryological development.
d. Tell hand out a sheet with the following inquiry questions to investigate:
a. Suppose an industrial accident occurred and released a teratogenic chemical
into the air. Women in a nearby community were breathing the air and some of
the women were pregnant. The teratogen absorbs into the lungs and will pass
along through the placenta. This teratogen is broken down and exits the body
within one week.
b. Evaluate any possible damage to the embryo for women who were at the
following stages of pregnancy:
i. 1 week
ii. 20 days
iii. 30 days
iv. 4 months
v. 6 months
vi. 8 months
e. Ask the student to be specific about the possible organ systems affected at that particular
time of development.
f. Ask the students if the results would be different if the chemical was stored in body fat
reserves for long periods of time.
LEARNING THROUGH SERVICE
Service learning is a strategy of teaching, learning and reflective assessment that merges the
academic curriculum with meaningful community service. As a teaching methodology, it falls
under the category of experiential education. It is a way students can carry out volunteer projects
in the community for public agencies, nonprofit agencies, civic groups, charitable organizations,
and governmental organizations. It encourages critical thinking and reinforces many of the
concepts learned in a course.
1. Have students volunteer at a reproductive services clinic.
2. Have students tutor high school students studying animal anatomy and physiology.
3. Have student develop a PowerPoint presentation on comparative embryology for local
high school teachers.
4. Have students produce human development educational materials in cooperation with
nurses and health education teachers at a local high school.
VISUAL RESOURCES
Models are extremely helpful to illustrate the movement of cells during gastrulation and
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neurulation. In a large class, color-coded transparencies will suffice. This may be a good time to
show a short film that shows the progression of development.
Bring in samples of various vertebrates in different stages to illustrate “ontogeny recapitulates
phylogeny.”
Obtain photos of human development at various stages. Be sure to specify the approximate size
of each stage. Most students are amazed to see the complexity of a three-month embryo, but are
more astounded when they are told that, or see that, it is only three centimeters long!
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