Essentials of Anatomy and Physiology

EMBRYO AND EMBRYONIC
MEMBRANES

An embryois the developing human individual from
the time of implantation until the eighth week of ges-
tation. Several stages of early embryonic development
are shown in Fig. 21–3. At approximately 12 days, the
embryonic disc(the potential person) is simply a
plate of cells within the blastocyst. As cells keep divid-
ing, the plate of cells begins to fold. At 14 days twin-
ning is believed to be no longer possible.
Very soon thereafter, three primary layers, or germ
layers, begin to develop: the ectoderm, mesoderm,
and endoderm. This is a major step in the specializa-
tion of the embryo cells, because different sets of
genes are switched off for each of the three layers,
while the genes appropriate for the layer remain
switched on. Each cell is now committed to a particu-
lar layer and can no longer “go back,” that is, become
a cell of either of the other two layers.
Each primary layer develops further; its cells
become more specialized as more and more genes in
each cell are switched off. With the active genes, those
that remain switched on, the cells in each layer are
thereby committed to develop into specific organs or

parts of organs. Ectomeans “outer”; the epidermis is

derived from ectoderm, and in some of these cells the

gene for producing keratin is active. Mesomeans “mid-

dle”; the skeletal muscles develop from mesoderm,

and in these cells the genes for producing myosin and

actin are active. Endomeans “inner”; the stomach lin-

ing is derived from endoderm, and in these cells, the

gene for producing pepsinogen is active. Table 21–1

lists some other structures derived from each of the

primary germ layers.

At 20 days, the embryonic membranescan be

clearly distinguished from the embryo itself. The yolk

sacdoes not contain nutrient yolk, as it does for bird

and reptile embryos. It is, however, the site for the for-

mation of the first blood cells and the cells that will

become spermatogonia or oogonia. As the embryo

grows, the yolk sac membrane is incorporated into the

umbilical cord.

The amnionis a thin membrane that eventually

surrounds the embryo and contains amniotic fluid.

This fluid provides a cushion for the fetus against

mechanical injury as the mother moves. When the

fetal kidneys become functional, they excrete urine

into the amniotic fluid. Also in this fluid are cells that

have sloughed off the fetus; this is clinically important

Human Development and Genetics 477

22Y

22X 22X

22X

Sperm

44XY 44XX

Male Female

Egg

(ovum)

Fertilized

egg

Sperm

Figure 21–1. Inheritance of gen-
der. Each ovum contains 22 auto-
somes and an X chromosome. Each
sperm contains 22 autosomes and
either an X chromosome or a Y
chromosome.
QUESTION: Which parent deter-
mines the sex of the embryo-fetus?