Download Reference for embryology

EMBRYONIC DEVELOPMENT
The process of development starts with the union of two haploid cells, the egg and the sperm to form the
fertilized egg or zygote. The zygote divides by ordinary mitosis into two cells, which each divide to form four and
so on, a process called cleavage, which results in the formation of an embryo. At first, division is rapid, each
succession group of cells being smaller than the ones before, so that the early embryonic stages, although
many-celled, are scarcely larger than the original egg itself. Development not only involves cell division but also
cell growth, which may affect the size of the cells only and not their structure, and cell differentiation, which
adapts the cells to special functions.
Sea Star Development
To see the developmental stages of the sea star we will use slides with all the stages of development from
the unfertilized egg to the late gastrula. Study your slide with low power, since the specimens can be easily seen
and drawn at this magnification.
Unfertilized egg: There are several of these scattered on the slide. They can be identified by their
spherical shape and their spherical nuclei. Note the small, dark nucleolus in the nucleus.
Fertilized egg (zygote): Find an egg with a very indistinct nucleus and nucleolus (or not visible). This is
a fertilized egg or zygote. The zygote will have a light-colored membrane, the fertilization membrane, around
them. This membrane usually rises up from the cytoplasm shortly after the entrance of the sperm into the egg.
Cleavage stages: Cleavage or cell division starts just after fertilization. The first mitotic division results
in two cells, which stay together. Now find a four-cell stage. The cleavage planes run through the poles,
perpendicular to each other. The fertilization membrane should still be visible at this stage.
Note that at each succeeding stage the total volume of the embryo does not exceed that of the early
one-cell stage. A mass of sixteen or more cells with no large cavity in its center is called a morula stage. How
does its diameter compare with the undivided zygote?
As cleavage continues, the cells become smaller and are finally arranged in a single layer (blastoderm)
around a central cavity, the blastocoel. This is the blastula stage. You can distinguish this stage by the regular
arrangement of the blastoderm and the difference between the dark outer zone and a lighter central region.
The embryo is now somewhat asymmetrical because one side of the blastula has been pushed in. At this
point the embryo is called a gastrula. This process involves unequal cell division with some migration of cells. In
the early gastrula stage, there is only a slight indentation where the pushing in occurs, but in later stages, it
continues to get deeper until the pushed-in side reaches the opposite side, thus forming a two-walled sac. The new
cavity is called the archenteron, and its opening to the outside is the blastopore. The outer layer of cells of the
gastrula is called the ectoderm; the inner layer, which forms the archenteron, is called the endoderm. The
ectoderm will give rise to the epithelium which covers the surface; the inner endoderm to the epithelial lining of
the digestive tube. As the archenteron enlarges, the old blastocoel becomes smaller and finally disappears.
The gastrula stage is as far as we will study the embryology of the sea star. In later stages a third layer, the
mesoderm, is formed between the other two. All structures in the body of the animal will come from these three
layers. The mesoderm gives rise to the great bulk of the body: skeleton, muscles, blood vessels, and reproductive
system. The ectoderm forms the outer layer of the skin, parts of sense organs, and the nervous system. The
endoderm forms the lining of the alimentary canal and parts of other organs derived from the digestive tube. The
archenteron remains as the anus of the larva. The gastrula then becomes a free-swimming bipinnaria larva which
develops into a brachiolaria larva and finally into a young sea star.
Assignment:
With a partner, view slide and draw representative stages (as seen on the board) of development
of the sea star and describe significant structures in the stage.
Be sure that you can identify each stage and and describe how the development is significant to
the formation of the three tissue layers that form the structures that make up the organism.