26.The answer is e.(Sadler, pp 12, 55–56, 58–59, 101, 203–206. Moore and
Persaud, Developing, pp 44–49, 142–143.)The yolk sac is important as a
source of blood until the fetal liver replaces this function in about the sixth
week of development. The yolk sac produces predominantly hematocyto-
blasts (stem cells) and primitive erythroblasts. Vasculogenesis is also initiated
in the yolk sac. The endoderm of the yolk sac is incorporated into the
embryo as part of the primitive gut during embryonic folding and is home to
the primordial germ cells before they migrate to the hindgut. There is no yolk
storage in human embryos (answer a).The transfer of nutrients is an impor-
tant function of the yolk sac early in development, but once the uteroplacen-
tal circulation is established, the placenta takes over that role (answer b).
The umbilical vein forms the ligamentum teres hepatis (answer c).The cells
of the amnion (answer d)form the amniotic fluid with eventual addition of
urine from the developing kidneys. The diagram above illustrates the loca-
tion of the yolk sac and other embryonic structures.
27.The answer is d.(Sadler, pp 104–107. Moore and Persaud, Developing,
pp 144–150. Alberts, p 1153.)Monozygotic (MZ) twins arise from divisions
of the embryoblast to form two embryos. MZ twins also form from early
Embryology: Early and General Answers 93(Modified, with permission, from Sweeney L. Basic Concepts in Embryology. New York, NY:
McGraw-Hill, 1998.)