Human Reproduction ❮ 205
Embryonic Development
Embryology,the study of embryonic development, is a detailed and complex field.
Fortunately, you are not taking an AP exam in embryology. Stick to the basics here and do
not let the complex details bog you down. Follow along with the pretty pictures, and the
review questions at the end of this chapter will give you a good indication of the level of
detail required for success on the embryology questions of the AP Biology exam.
Cleavage
Embryonic development begins as soon as the egg is fertilized to produce a diploid zygote
(2n). This zygote then divides mitotically many times without increasing the zygote’s over-
all size. During these “cleavage” divisions(Figure 16.5), cytoplasm is distributed unevenly
to the daughter cells but genetic information is distributed equally. This disparity exists
because different cells will later produce different final products and the uneven distribu-
tion of cytoplasm plays a role in that process.
These cleavage divisions take a while in humans. The first three divisions take three days
to complete. After the fourth division, the one cell has become 16 cells and is now called a
morula.As it undergoes its next round of cell divisions, fluid fills the center of the morula
Figure 16.5 Embryonic cleavage divisions. (FromBiology,8th ed., by Sylvia S. Mader, © 1985, 1987,
1990, 1993, 1996, 1998, 2001, 2004 by the McGraw Hill Companies, Inc. Reproduced with permission of
The McGraw-Hill Companies.)
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