HUMAN BIOLOGY

Development anD aging 339

fetus A term used to desig-

nate a human embryo after

it has completed 8 weeks of

development.

hoW does an embryo change during the

second 4 Weeks of development?

• During its second 4 weeks, an embryo begins to take on the
  appearance of a human as features of its head, face, limbs,
  and genitals develop.

taKe-Home message

As the second half of the first trimester begins, gonads
begin to develop. In an embryo that has inherited X and
Y sex chromosomes, a sex-determining portion of the
Y chromosome now triggers development of testes (Fig-
ure 17.13). Sex hormones made by the testes then influ-
ence the development of the entire reproductive system.
An embryo with XX sex chromosomes will be female,
and female reproductive structures begin to form in her
body. Notice that no hormones are required to stimulate

Figure 17.13 Genitals of all embryos start out the same.

The male sex hormone testosterone must be present in

order for male genitals to develop. (© Cengage Learning)

7 weeks

10 weeks

Y chromosome

present

Y chromosome

absent

birth approaching birth approaching

penis

vaginal

opening

WEEK 8

C

final week of embryonic

period; embryo looks

distinctly human

compared to other

vertebrate embryos

upper and lower limbs well

formed; fingers and then

toes have separated

early tissues of

all internal, external

structures now developed

tail has become stubby

actual length

Lennart Nilsson from

A Child Is Born

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

development of female gonads—all

that is necessary is the absence

of testosterone.

After 8 weeks the embryo is just

over 1 inch long, its organ systems

are formed, and it is designated a fetus. As the first tri-

mester ends, a heart monitor can detect its heartbeat. Its

genitals are well formed, and an ultrasound image often

will reveal the baby’s sex.

© Cengage Learning

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