Download Survey of Animal Development Instructions

Survey of Animal Development
Instructions
Activity 1. Observing Early Development in the Starfish
The starfish is used to illustrate early development because the events of cleavage and gastrulation are especially easy
to see. More complex development patterns in other organisms may be compared to this simple pattern.
Procedure
1.
Obtain a microscope slide containing developmental stages of the starfish from the unfertilized egg through
the gastrula stages.
2.
Scan the slide under low power with the compound microscope and locate the stages described in this section
and shown in Fig. 1. These slides are usually thicker than most slides you have studied. Do not use high power,
or you may push the objective through the coverslip while focusing.
Fig. 1. Stages in starfish development: (a) unfertilized egg; (b) zygote surrounded by fertilization membrane; (c) two-cell
stage (blastomeres); (d) four-cell stage; (e) morula; (f ) blastula; (g) early gastrula; (h) late gastrula.
•
An unfertilized egg contains a single egg nucleus and a large amount of rather equally distributed yolk
surrounded only by a cell membrane.
•
When the sperm penetrates the egg, a fertilization membrane forms as materials stored beneath the
cell membrane are released. This new membrane may be visible as a "halo" above the surface of
fertilized eggs on your slide. It acts as a barrier that prevents the penetration of additional sperm. Why
is this important? Think of the genetic consequences of polyspermy: the fusion of more than one sperm
with an egg.
Find examples of unfertilized and fertilized eggs on the slide, and check them off (!) on Fig. 1 when
you observe them.
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3.
For several hours following fertilization, the zygote undergoes a series of rapid mitotic cell divisions without
any intervening periods of growth. This cleavage process involves the duplication of chromosomes followed
immediately by mitosis and cytokinesis, followed again by chromosome duplication, followed by mitosis, and
so on. The result is that the large fertilized egg is divided into smaller and smaller cells, each with a single
nucleus.
Answer Questions 1 and 2 on the Lab Report.
4.
The cleavage divisions are usually synchronized in all cells, so that the embryo goes through a series of stages
in which it contains at first 1 then 2, 4, 8, 16, 32, 64 cells, and so on. Around the 32-cell stage, the number of
cells becomes difficult to count, and the developing embryo is referred to as a morula, meaning mulberrylike.
Note that the size of the developing embryo does not increase as cell number increases. The existing material
of the embryo is merely partitioned by cleavage into smaller cells.
Find examples of 2-, 4-, 8-, and 16-cell stages on your slide, checking them off of Fig. 1.
5.
As cleavage continues. the cells eventually form a hollow ball of cells that is ciliated on its outer surface. The
beating of the cilia allows the embryo to swim alit of the enveloping fertilization membrane. This stage is
called a blastula, the cells are blastomeres, and the central cavity is the blastocoel. Find a blastula on your slide
and identify these structures.
Check off the blastula on Fig. 1. Answer Question 3 on the Lab Report.
6.
Several hours after blastula formation, a second major morphogenic event occurs. Cells at one end of the
blastula undergo rapid growth and move into the blastocoel. This infolding is called invagination or
gastrulation and results in a two-layered embryonic stage called the gastrula (Fig. 1g and 1h). The gastrula then
elongates, forming a tube-within-a-tube cylindrical body.
The two layers of cells in the gastrula are called primary germ layers. As development continues, certain tissues
and organs will form from one or the other of these layers. The inner layer, called the endoderm, will form the
lining of the digestive system and digestive glands. The outer layer is the ectoderm and will form the skin and
the nervous system of the adult. The midgastrula and late gastrula stages can be identified by the elongation of
the gastrula and changes in the shape of the endoderm tube. The elongated tube is called the archenteron,
meaning primitive gut, and its opening to the outside is called the blastopore. It will become the anus of the
starfish. The end of the archenteron away from the blastopore eventually develops two lateral pouches that will
grow outward and pinch off, forming a third germ layer called the mesoderm. The start of this process is
shown in Fig. 1h. Muscles, connective tissues, and gonads will develop from the cells in this third layer.
Find a late gastrula stage on your slide and identify all the structures indicated by boldface terms in
this paragraph. Check off the late gastrula in Fig. 1.
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In the transition from blastula to gastrula, a lot has happened. The basic shape of an animal has appeared.
Most animals have a digestive tube-within-a-body-tube plan of organization, if you neglect the appendages.
Think of the bodies minus appendages for an earthworm, insect, and human to test this idea. In addition, the
gastrula stage is the first time we see distinct tissues, and these will further develop to yield the many tissues
found in an adult.
Beyond the gastrula stage, differentiation continues. Most animals, except for birds and mammals, have a free
living larval stage. A larva is capable of feeding itself and continues to grow, using external energy sources. At a
later stage, a metamorphosis occurs, and the larva changes to a miniature version of the adult starfish.
Activity 2. Observing Mid-Development in the Chick
We will now begin to explore later stages of development in
the chick embryo. As you are probably aware, chick
embryos develop within the protective confines of a hardshelled egg. Most eggs that are sold for human
consumption are from hens that are not exposed to roosters
and are therefore unfertilized. If you were to get a fertilized
chicken egg, you would find the embryo growing on the
surface of the egg yolk. In its early stages, it would resemble
a disc of tissue.
Of the many aspects of chick development that can be
observed in well-prepared specimens is the formation of
mesoderm following gastrulation. The mesoderm in chicks
(as well as in many other vertebrates) appears as mounds or
bundles of tissue called somites, which will form (1) the
cartilage of the vertebrae and ribs, (2) the muscles of the rib
cage, limbs, and back, and (3) the dermis of the dorsal
(back) skin. Chicks at specific stages of development have
characteristic numbers of somites.
Fig. 2. Note the somites of this chick embryo as paired
mounds of tissue along the length of the neural tube.
Procedure
1.
View slides of chick embryos and count the numbers of somites at each stage. When viewing these slides,
please only use the 4x and 10x objective lenses. As you can see, these slides are much thicker than others we
have viewed this semester.
Answer Questions 4-6 on the Lab Report.
2.
Observe the biomount that contains several vertebrate embryos.
Answer Questions 7-8 on the Lab Report.
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Activity 3. Observing Late Development in the Human
1.
Study Figure 3 below until you understand the location of the extra-embryonic membranes.
Fig. 3. Extraembryonic membranes of the human (and most other mammals).
2.
Study Figure 4a, noting the formation of the extra-embryonic membranes and germ layers.
Answer Question 9 on the Lab Report.
3.
Examine Figure 4b, noting the components of the umbilical cord, the placenta, and the amniotic fluid
enveloping the fetus.
4.
Examine the pregnant human pelvis model and compare it with Figure 4c. Note how the amnion and chorion
are pressed together, forming the amniochorion in late stages of pregnancy.
Answer Questions 10-11 on the Lab Report.
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Fig. 4. Selected stages in human development. (a) A human embryo, about 14 days old, implanted in the uterus. Note the
germ layers and extraembryonic membranes. (b) Uterus with a fetus about 10 weeks old. Note the extraembryonic membranes
and the umbilical cord. (c) A fullterm fetus with head pressed against the cervix. Note the placenta, umbilical cord, and the
fetal position.
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Survey of Animal Development
Lab Report
Name ________________________________________
Activity 1. Observing Early Development in the Starfish
1.
In starfish, the diploid chromosome number (2n) is 36. How many chromosomes would you expect to find in
an unfertilized starfish egg? ______________
2.
The fertilization envelope forms around the newly-fertilized egg to prevent a condition called polyspermy.
How many chromosomes would you predict you’d find in an egg that had a defect in its fertilization
membrane and was fertilized by two sperm? Why?
3.
If the type of cell division is mitosis, do the nuclei of the developing embryo contain the same or different
genetic information than the zygote's nucleus?
Activity 2. Observing Mid-Development in the Chick
4.
Fill in the table below with the number of somite pairs you can see in the following embryos.
Stage
# Somite Pairs
16-hr
18-hr
24-hr
27-29-hr
48-hr
56-hr
5.
At what stage do the somites first appear?
_______________________
6.
At what stage does the chick possess all of its somites? _______________________
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7.
Which of the embryos in the Biomount most closely resembles what the adult form will look like? Why?
8.
List some physical characteristics that these embryos have in common.
Activity 3. Observing Late Development in the Human
9.
In the figure above, color the three germ layers as instructed below:
ectoderm—green
mesoderm—red
endoderm—yellow
10. Does the fetus fill all available space?
______________
11. What is the gender of the fetus in the pregnant pelvis model?
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______________