12.The answer is d.(Moore and Persaud, Developing, pp 46–50, 74. Sadler,
pp 45–47, 63–64.)During the second week of fetal development, lacunar
spaces develop between cells of the syncytiotrophoblast, particularly in the
region of the embryonic pole as the conceptus invades the endometrium.
Endometrial capillaries in this region become dilated and engorged with
blood to form sinusoids. The syncytial cells direct erosion of the endothelium
of the maternal capillaries, allowing maternal blood to enter the lacunae and
bathe the syncytial cells. During the second week, primary villi consist of
projections of syncytial cells surrounding a core of cytotrophoblast cells
(answer b).During the third week (answer c),the villus core is invaded by
mesodermal cells to form a secondary villus. Cells of the mesodermal core will
then differentiate to form capillaries and blood cells by the end of the third
week (tertiary villus). Those vessels become connected to the fetal circulation
early in the fourth week establishing the functional uteroplacental circulation.
13.The answer is e.(Sadler, pp 45–46, 48, 55–56, 58. Moore and Persaud,
Developing, pp 63–64.)The nongastrulating cells of the epiblast form the
ectoderm (epidermis, epidermal appendages, and the nervous system).
During the second week of development, the embryoblast gives rise to two
primitive germ layers, the epiblast and the underlying hypoblast (answer a).
At the beginning of the third week, cells from the epiblast (answer b)
migrate toward the midline (primitive streak) and move inward (gastrula-
tion). The migrating epiblast cells displace the hypoblast cells to the
periphery to form the endodermal lining (answer d)of the digestive tract
and form an intermediate layer of mesoderm (answer c)that will give rise
to muscle, bone, and cartilaginous structures.
14.The answer is b.(Moore and Persaud, Developing, pp 372–373. Sadler,
pp 189–192.)Blood from the placenta in the umbilical cord is about 80% oxy-
genated. Mixture with unoxygenated blood from the vitelline veins and the
inferior vena cava reduces the oxygen content somewhat. However, this
stream with relatively high oxygen content is directed by the valve of the infe-
rior vena cava directly through the foramen ovale into the left atrium. This
prevents admixture with oxygen-depleted blood entering the right atrium
from the superior vena cava. Thus, the oxygen-saturated blood entering the
left ventricle and pumped into the aortic arch, subclavian arteries, and com-
mon carotid arteries has the highest oxygen content. The oxygen-depleted
blood from the superior vena cava is directed into the right ventricle and then
Embryology: Early and General Answers 85