HUMAN BIOLOGY

334 Chapter 17

embryonic disk Pancake-
shaped stage of an early
embryo.

neural tube The forerunner
of an embryo’s brain and
spinal cord.

How the early embryo Develops

On the surface of the embryonic disk near the neural

tube, the other primary tissue layer—mesoderm—also

has been forming. Toward the end of the third week,

some mesoderm gives rise to somites (soe-mites). These are

paired blocks of mesoderm, and they will be the source of

most bones and skeletal muscles of the neck and trunk.

The dermis overlying these regions comes from somites

as well. Structures called pharyngeal arches start to form

(Figure 17.7C). They will contribute to development of the

face, neck, mouth, and associated parts. In other meso-

dermal tissues, spaces open up. Eventually, these spaces

will merge to form the cavity (called the coelom, see-lahm)

between the body wall and the digestive tract.

n    The embryonic period lasts for 8 weeks. during that time,

the basic body plan of the embryo takes shape.

A developing baby is considered an embryo for most of the

first trimester, or 3 months, of the 9 months of gestation.

When the three germ layers—the ectoderm, mesoderm,

and endoderm—are in place, morphogenesis begins and

the embryo’s organ systems start to develop.

First, a basic body plan is established

By the time a woman has missed her first menstrual period,

the embryo has implanted and the inner cell mass has been

transformed into a pancake-shaped

embryonic disk. Around day 15, gas-

trulation has rearranged cells so that

a faint “primitive streak” appears at

the midline of the disk (Figure 17.7A).

Now, ectoderm along the midline

thickens to establish the beginnings

of a neural tube (Figure 17.7B). This

tube is the forerunner of the embryo’s brain and spinal cord.

Some of its cells also give rise to a flexible rod of cells called

a notochord. The vertebral column will form around this rod.

These events establish the body’s long axis and its

bilateral symmetry—the more-or-less matching halves on

either side of the long axis. In other words, the embryonic

disk is reshaped in ways that provide the body with the

basic form we see in all vertebrates.

F i g u r e 17.7 Several important steps mark the embryonic period of development. These steps include the appearance of a primitive

streak foreshadowing the brain and spinal cord and the formation of somites and pharyngeal arches. These are dorsal views (of the embryo’s

back) except for days 24–25, which is a side view. (© Cengage Learning)

C Days 24–25. By now, some

cells have given rise to pharyngeal

arches, which contribute to the

face, neck, mouth, nasal

cavities, larynx, and pharynx.

yolk sac

embryonic disk

amniotic cavity

chorionic cavity

primitive streak

neural

tube

pharyngeal

arches

future brain

somites

B Days 19–23. Cell migrations, tissue folding,

and other morphogenetic events lead to the

formation of a hollow neural tube and to

somites (bumps of mesoderm). The neural

tube gives rise to the brain and spinal cord.

Somites give rise to most of the axial skeleton,

skeletal muscles, and much of the dermis.

A Day 15. A primitive streak

appears along the axis of the

embryonic disk. This thickened

band of cells marks the onset of

gastrulation.

lined body cavity (coelom); lining

also holds internal organs in place

peritoneum

epidermis gut cavity

1 7. 4

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