Answers to Review Questions
Chapter 60 (p. 1238)
1. The three stages of fertilization are (1) penetration, when the sperm actually gets through the outer protective layer around the egg; (2) activation, which prevents other sperm from entering the egg, induces the egg to undergo its second meiotic division, and stimulates cytoplasmic movements; and (3) fusion, during which the haploid nuclei of the sperm and egg fuse to form a diploid zygote.
2. Holoblastic cleavage is characterized by little yolk, a symmetrical blastula, cells that are all the same size in fish. In amphibians there is a lot of yolk, asymmetrical cleavage, large yolk-containing cells at one end, and small cells with less yolk at the other. Reptiles and birds are characterized by meroblastic cleavage in which the egg is mostly yolk, cleavage occurs only through the blastodisc, the blastoderm is not spherical, but a hollow cap atop the yolk. In mammals there is holoblastic cleavage with little yolk, the blastula mass is concentrated at one end (analogous to blastodisc), and the remaining region is a trophoblast, which forms placental membranes.
3. Gastrulation causes the animal hemisphere of the blastula to bulge inward, forming a cup-shaped embryo. The hollow crater resulting from the invagination is the archenterons. This develops into the anus in vertebrates.
4. Gastrulation in amphibians differs in that it is not mechanically possible to invaginate at the vegetal pole due to the few, large, yolk-laden cells. This changes the cell migration pattern so that the layer of animal pole cells folds over the yolk cells and invaginates inward. The dorsal lip of the gastrula is the region at which animal cells invaginate. The yolk plug is a region of the blastopore that is filled with yolk-rich cells.
5. Only chordates have neurulation. The two unique structures are the notochord and the hollow dorsal nerve cord.
6. The amnion surrounds the embryo and the amniotic fluid in both birds and mammals. In birds, the chorion is next to the eggshell. The allantois stores uric acid wastes, growing larger through development. Eventually it fuses with chorion and functions as a "lung" to exchange gases between the embryo and the external environment. In mammals, the part of the chorion that contacts the endometrium becomes the fetal placenta. The allantois contributes blood vessels to the developing umbilical cord.
7. The placenta secretes HCG, which maintains the placenta during the first two months of pregnancy.
8. Organogenesis occurs in the fourth week. This is the period when spontaneous abortions can be caused by chemicals or other events.
9. Neurological growth is not complete at birth; however, remaining in utero longer to further neurological development would also result in an increase in bulk, and the baby would grow too large to pass through the birth canal.
10. Lactation is stimulated by prolactin. Milk ejection from the breast is stimulated by oxytocin.