DEVELOPMENT
Development begins with the union of an egg and a sperm.
Examine a slide labeled "Ovary" and identify an egg. Examine a sperm
smear and identify normal sperm. Sketch below.
Early stages of human development are common to lower animals
as well. Starfishes, for example, can be used to study fertilization, cell
cleavage, and the developmental stages designated morula, blastula,
and gastrula, so slides of starfish and sea urchin development will be
available for microscopic examination.
Fertilization is the union of a sperm with an egg for the purpose
of introducing paternal chromosomes into the egg. An example of an
unfertilized starfish egg is shown below. Following fertilization, the egg
begins to divide in order to generate the large number of cells that
constitute the embryo. One cell yields two; these divide to yield four,
etc., so that 2-, 4-, 8-, 16-, and 32-cell cleavage stages are generally
seen in development. The result of cleavage is a tightly-packed mass of
cells, a morula, which usually consists of 16 or 32 cells. Examples of
cleavage stage embryos and a morula are shown below. Study these
figures, then identify a fertilized egg, cleaving embryos, and a morula on
a "Starfish Development" slide.
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Does the starfish embryo grow larger as it progresses from the egg
to the morula stage? Why/why not? Do you think the same
would be true of an early human embryo? Why/why not?
At what site would you expect to find a fertilized human egg? a
morula? (See Figure 18.3 in the Mader text.)
The cells of the morula continue to divide, and a fluid-filled space
begins to develop in the center of the cell mass, resulting in the stage
known as the blastula in animals or blastocyst in a human embryo. As
cells continue to migrate to the periphery of the mass, a thin outer layer,
the ectoderm, forms. Then, from a pre-determined point on the
surface, cells begin a complex migration into the blastocoel (the central
fluid cavity) to form an inner layer of cells, the endoderm. Eventually,
the endoderm will give rise to a third, middle layer, the mesoderm. The
mesoderm cannot be seen on the "Starfish Development" slides, but
you should be able to find blastulae and the ectodermal and
endodermal layers of a gastrula, the stage resulting from the inward
migration of the peripheral cells. Refer to the figures below as a guide.
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Early gastrula
Once the three germ layers of the embryo have formed,
differentiation begins, i.e., cell fate becomes determined. Cells of the
ectoderm form the epidermis and central nervous system, the
mesoderm gives rise to many internal organs and connective tissues,
and the endoderm gives rise to much of the digestive tract. Refer to
Table 18.1 in Mader for further examples.
Details of development obviously differ from one organism to
another, but the stages which you have seen in the starfish are common
to most animals, including humans.
Internet Resources
To see an animated version of sea urchin development, go to
http://www.stanford.edu/group/Urchin/urchin.html
To see the animated developmental stages of another
experimental organism (Drosophila, the fruitfly), connect to the site at
http://flybase.bio.indiana.edu/images/lk/Animation/
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