by which spermatogonia undergo mitotic division to produce primary
spermatocytes, occurs at 1°C (2°F) below normal body temperature. Sub-
sequent meiotic divisions produce secondary spermatocytes with a biva-
lent haploid chromosome number and then spermatids with a monovalent
haploid chromosome number. Spermiogenesis, the maturation of the sper-
matid, results in spermatozoa. Morphologically, adult spermatozoa are
moved to the epididymis, where they become fully motile.
9.The answer is b.(Moore and Persaud, Developing, pp 47–51, 52–54. Sadler,
pp 41, 42, 49–52.)Implantation of the conceptus low on the uterine wall near
the cervical opening (os) could result in growth of the placenta between the
embryo and the cervical os (placenta previa). The placenta could become dis-
lodged from the uterine wall before, as well as during delivery, resulting in
rapidly fatal hemorrhage. Implantation at site A (the uterine tube or oviduct)
results in rupture of the oviduct wall, whereas implantation on the ovary (C)
would result in destruction of that organ. Implantation could also occur in
the wall of the peritoneal cavity (D). Implantation normally occurs in the
superior posterior or posterolateral walls of the uterus (E).
10.The answer is b.(Moore and Persaud, Developing, pp 44–46. Sadler, pp
39–40, 45–46, 48, 55–56, 58.)Cells of the inner cell mass (embryoblast) of
the blastocyst differentiate into the epiblast and hypoblast. Cells of the epi-
blast migrate toward the primitive streak during the second week and
become internalized, forming the mesodermal and endodermal germ lay-
ers. Remaining cells of the epiblast become the ectodermal germ layer.
Cells of the hypoblast (answer d)will contribute to the yolk sac. Cells of
the outer cell mass of the blastocyst will differentiate into the cytotro-
phoblast and syncytiotrophoblast (answers a and c),which will con-
tribute to formation of the placenta. The yolk sac (answer e)is incorporated
into the embryo as the primitive gut during embryonic folding.
11.The answer is b.(Moore and Persaud, Developing, pp 88–90, 520.)For-
mation of most internal organs occurs during the second month, the period
of organogenesis. The first month (answer a)of embryonic development
generally is concerned with cleavage, formation of the germ layers, and
establishment of the embryonic body. The period from the ninth week to the
end of intrauterine life (answers c, d and e),known as the fetal period, is
characterized by maturation of tissues and rapid growth of the fetal body.
84 Anatomy, Histology, and Cell Biology