206 ❯ STEP 4. Review the Knowledge You Need to Score High
to create the hollow-looking structure known to embryologists as the blastula.The fluid-filled
cavity in the blastula is known as the blastocoel. Up to this point, much of the dividing has
occurred as the zygote moves toward the uterus through the fallopian tube. By the time the
blastula has formed, it has reached the uterus and has implanted on the wall. The blastula con-
tains two parts: an inner cell mass,which later becomes the embryo, and a trophoblast,
which becomes the placenta for the developing fetus and aids in attachment to the
endometrium. The trophoblast also produces human chorionic gonadotropin(hCG), which
maintains the endometrium by ensuring the continued production of progesterone and estro-
gen. The trophoblast later gives rise to the chorion, which we will discuss later.Gastrulation
Okay, here’s where the discussion of embryology gets a little bit tricky. The next major
stage of embryonic development after cleavage is gastrulation(also called morphogenesis).
During gastrulation, cells separate into three primary layers called germ layers,which even-
tually give rise to the different tissues of an adult.
Let’s look at this process in a bit more detail. (See also Figure 16.6.) After the blas-
tocyst attaches to the uterine wall, the inner cell mass divides into two major cell masses:Figure 16.6 Components in the gastrulation process. (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.)