894
UNIT SIX
malities too severe to maintain life.
Table 23.3
summarizes
the stages of prenatal development.
PRACTICE
21
What major changes happen during the fetal stage of
development?
22
When can the sex of a fetus be determined visually?
23
How is a fetus usually positioned in the uterus as birth nears?
nails; and the skull bones are largely ossiF ed. As
f gure 23.23
shows, the fetus is usually positioned upside down with its
head toward the cervix (
vertex position
).
The birth of a live, healthy baby is against the odds, con-
sidering human development from the beginning. Of every
100 secondary oocytes exposed to sperm, eighty-four are fer-
tilized. Of these, sixty-nine implant in the uterus, forty-two
survive one week or longer, thirty-seven survive six weeks
or longer, and only thirty-one are born alive. Of those that
do not survive to birth, about half have chromosomal abnor-
Genital tubercle
Urogenital folds
Labioscrotal folds
Fused urogenital folds
Perineum
Anus
Urethral groove
(a)
Urogenital fold
Developing penis
(c)
(e)
(d)
(f)
Glans penis
Scrotum
Genital tubercle
Glans
Urogenital fold
Labioscrotal fold
(b)
Developing clitoris
Embryonic tail
Urethral groove
Glans clitoris
Hymen
Vaginal
orifice
Labia minora
Labia majora
Perineum
Anus
Urethral orifice
Prepuce
Male
Female
FIGURE 23.22
Formation of external reproductive organs. (
a
and
b
) The genital tubercle, urogenital fold, and labioscrotal folds that appear
during the fourth week of development may diĀ±
erentiate into (
c
and
d
) male external reproductive organs or (
e
and
f
) female external reproductive
organs.
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