W e e k 2 o f d e v e l o p m e n t .
Successful im-
plantation provides a safe environment for the
embryo, which will spend the next week pene-
trating the endometrium to make connections
to the uterine blood supply to obtain nutrients,
exchange gases, and eliminate wastes. These
connections are made through the formation
of
chorionic v illi,
which are finger-like projec-
tions from the blastocyst to the tissues of the
mother’s uterus (Figure 16.13a). Oxygen and
nutrients in the mother’s blood diffuse across
the cell membranes into the capillaries of the
chorionic villi. Waste products such as carbon
dioxide diffuse in the opposite direction.
Also at this time, the embryo begins se-
creting the hormone human chorionic go-
nadotropin (hCG); hCG prevents destruction
of the corpus luteum in the ovary, allowing con-
tinued secretion of the estrogens and proges-
g a s tr u la tio n
(gas'-
troo-LA-shun) The
migration of groups of
cells from the epiblast
(primitive ectoderm) that
transform a bilaminar
embryonic disc into a tri-
laminar embryonic disc
consisting of the three
primary germ layers;
transformation of the
blastula into the gastrula.
g e r m la y e r s
The
three major embryonic
tissues (ectoderm, me-
soderm, and endoderm)
from which the various
tissues and organs of
the body develop.
terone that maintain the mother’s body in a
pregnant state (that is, maintaining the uter-
ine lining and suppressing menses).
Embryonic Germ Layers
Differentiate to Form the
Various Body Components
W e e k 3 o f d e v e l o p m e n t .
Beginning in
the third week, the embryo undergoes many
changes in shape and complexity. Cells within
the flat, two-layered embryonic disc migrate
inward to become a three-layered structure in
a process called
g a s t r u la t io n
. The three layers
of cells are called the
g e r m la y e r s
and consist
of the ectoderm, mesoderm, and endoderm.
These layers later differentiate into various
organs.
C o n n e c t i o n s b e t w e e n t h e e m b r y o a n d t h e m o t h e r
F i g u r e 1 6 . 1 3
a.
Formation of chorionic villi
Chorionic villi
Amniotic cavity
Yolk sac
Amniotic cavity
Embryo
--------
Yolk sac
Chorionic
villus
Frontal section through uterus showing
blastocyst, about 13 days after fertilization
Frontal section through uterus showing embryo and
vascular supply, about
2 1
days after fertilization
490 CHAPTER 16
The Reproductive Systems
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