HUMAN BIOLOGY

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332 Chapter 17

(Figure 17.3D). The blastocyst
has two tissues: a surface epi-
thelium called the trophoblast
(tropho- means “to nourish”)
and a small clump of cells
called the inner cell mass
(Figure 17.3E). The embryo
develops from the inner cell
mass.
Sometimes a split separates the two cells produced by the
first cleavage, the inner cell mass, or an even later stage. Then,
separate embryos develop as identical twins, who have the
same genetic makeup. Fraternal twins result when two eggs
are fertilized at roughly the same time by different sperm.
Fraternal twins need not be the same sex, and they don’t
necessarily look any more alike than other siblings do. Focus
on Health on the facing page looks at health issues that may
arise with twinning.

Implantation secures the embryo in the uterus


About a week after fertilization, implantation begins as
the blastocyst breaks out of the zona pellucida. Cells of the
blastocyst’s epithelium then invade the endometrium (the
lining of the uterus) and cross into the underlying connec-
tive tissue (Figure 17.4). This gives the blastocyst a foothold

blastocyst The embryonic
stage that develops from
a morula and eventually
implants in the uterine wall.


embryo The stage of a
newly forming individual that
arises from the inner cell
mass of a blastocyst.


implantation Attachment of
a blastocyst to the uterine
lining (endometrium).


inner cell mass The mass
of cells in a blastocyst that
develops into an embryo.


n    A newly formed embryo cannot survive unless it implants in
the mother’s uterus.
n Link to Fertilization 16.6

The previous section gave you an overview of the basic
processes of early development. In the next few sections we
look in more depth at how these processes culminate in a
newborn baby.

Cleavage produces a multicellular embryo


As you’ve read, the zygote spends several days moving
down the oviduct before it reaches the mother’s uterus.
During this time it is sustained by
nutrients from the ovum or from
substances secreted by the mother’s
tissues. On the way, the three cleav-
ages described in Section 17.1 occur,
converting the single-celled zygote
into a morula (Figure 17.3, stages
A– C).
When the morula finally reaches
the uterus, a cavity filled with fluid
begins to open up inside it. This
change transforms the morula into
a blastocyst (blast- means “bud”)

From zygote to implantation


endometrium

actual
size

inner cell mass

E Days 6–7.
Some of the blastocyst’s
surface cells attach
themselves to the
endometrium and start to
burrow into it. Implantation
has started.

D Day 5.
A fluid-filled cavity forms in
the morula. By the 32-cell
stage, differentiation is
occurring in an inner cell
mass that will give rise to
the embryo. This embryonic
stage is the blastocyst.

C Day 4.
By 96 hours there is a ball
of 16 to 32 cells. This is the
morula. Cells of the surface
layer will function in
implantation and will give
rise to a membrane, the
chorion.

inner cell mass

fertilization

implantation

blastocoel

fluid

endometrium

uterine
cavity

B Day 3.
After the third
cleavage, cells
form a compact
ball.

trophoblast (surface
layer of cells of the
blastocyst)

A Days 1–2.
The first cleavage
furrow extends
between the two
polar bodies.

Figure 17.3 Animated! Implantation begins about a week after an egg is fertilized. (© Cengage Learning)

1 7. 2


Identical twin sisters

Dennis Degnan/Flirt/Corbis

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