HUMAN BIOLOGY

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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