94 Anatomy, Histology, and Cell Biology
separation of the blastomeres. Basically, MZ twins can arise anywhere from
the two-cell (blastomere) to the morula stage. MZ twins may have fused or
separate placentae, separate or fused dichorionic sacs or one chorionic sac,
and diamniotic sacs. Dizygotic (DZ) twins arise from fertilization of two
oocytes by two sperm (answer b)and are merely “womb mates.” They dif-
fer in genotype and, therefore, may be different sexes. Fertilization of one
oocyte by two sperm (answer c)cannot occur because of the Ca^2 +-depen-
dent block to polyspermy (see question 6). That egg cortical reaction
affects the zona pellucida in two ways: (1) hydrolysis of carbohydrate pre-
vents sperm binding and (2) proteolytic activity hardens it.
28.The answer is b.(Sadler, pp 63–72, 80. Moore and Persaud, Developing, pp
125–142.)Each of the embryonic germ layers is continuous with an extraem-
bryonic structure (see figure below). Ectoderm is continuous with the amniotic
membrane, endoderm with the lining of the yolk sac (answer c),and embry-
onic mesoderm with the extraembryonic mesoderm (answer d).The chorion
(answer a)consists of two parts, smooth (laeve) and villous (frondosum; see
figure in feedback to question 25.) The villous chorion attaches to the decidua
basalis of the placenta. There is no tunica adventitia in the umbilical vessels
(answer e).
(Modified, with permission, from Sweeney L. Basic Concepts in Embryology. New York, NY:
McGraw-Hill, 1998.)