HUMAN BIOLOGY

338 Chapter 17

the second Four Weeks: Human Features appear

becomes more intricate, and the umbilical cord forms.

Growth of the all-important head now surpasses that of

any other body region (Figure 17.12A and 17.12B). The

embryonic period ends as the eighth week draws to a

close. The embryo is no longer merely “a vertebrate.” As

you can see in Figure 17.12C it now clearly looks like a

human being.

n By the end of 4 weeks, the embryo has grown to 500 times

its original size. Over the next several weeks it will develop

recognizable human features.

In an embryo’s first few weeks of life, it grows rapidly

and its cells begin to specialize. Morphogenesis begins

to sculpt limbs, fingers, and toes. The circulatory system

F i g u r e 17.12 Amazing photographs show the development of a human embryo. A Human embryo at 4 weeks. It has a tail and

pharyngeal arches, which all vertebrate embryos have. B The embryo at 5 to 6 weeks after fertilization. C An embryo at the

boundary between the embryonic and fetal periods. It is floating in fluid within the amniotic sac. The chorion, which normally

covers the amniotic sac, has been opened and pulled aside. (© Cengage Learning)

WEEK 4

A

yolk sac

connecting stalk

embryo

forebrain

future lens

pharyngeal

arches

developing heart

upper limb bud

somites

neural tube

forming

lower limb bud

tail

actual length actual length

head growth exceeds

growth of other regions

retinal pigment

future external ear

foot plate

upper limb differentiation

(hand plates develop, then

digital rays of future fingers;

wrist, elbow start forming)

umbilical cord forms

between weeks 4 and 8

(amnion expands, forms

tube that encloses the

connecting stalk and a

duct for blood vessels)

WEEKS 5– 6

B

Lennart Nilsson from

A Child Is Born

, © 1966, 1977 Dell Publishing Co., Inc.

Lennart Nilsson from

A Child Is Born

, © 1966, 1977 Dell Publishing Co., Inc.

1 7. 7

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