1. The three stages of fertilization are penetration, when the sperm actually gets through the outer protective layer around the egg; activation, which prevents other sperm from entering the egg, induces the egg to undergo its second meiotic division, and stimulates cytoplasmic movements; and 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, 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 not spherical, but a hollow cap atop yolk. In mammals there is holoblastic cleavage with little yolk, the blastula mass concentrated at one end (analogous to blastodisc), 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 archenteron. The opening is the blastopore. 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. The mesoderm forms from cells of the dorsal lip that migrate between the ectoderm and endoderm.
5. Only chordates have neurulation. The two unique structures are the notochord and the hollow dorsal nerve cord. They arise from mesoderm and ectoderm, respectively. Neural crest is derived from cells from the neural groove. This tissue develops into forms that merge with the forebrain, becoming gill arches, Schwann cells, the adrenal medulla, sense organs, and the skull.
6. The chorionic frondosum and the decidua basalis together form the placenta. The placenta secretes HCG, which maintains the placenta during the first two months of pregnancy. By the end of two months, the placenta is secreting its own estradiol and progesterone to maintain itself; HCG secretion begins to dwindle.
7. Gastrulation occurs in the second week; neurulation in the third week; and organogenesis in the fourth week. This entire period is critical because the stage is set for the entire development of the organism, chemicals can have a great effect of development and can upset organogenesis, and this is the period when spontaneous abortions can be caused by chemicals or other events.
8. During the second trimester there is continued growth and development, bone enlargement, the development of body hair, and the development of a heartbeat. Survival would not be possible without special medical intervention. During the third trimester primarily growth occurs, with most development completed. The neurological growth includes formation of the major nerve tracts and new brain cells. 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.
9. Lactation is stimulated by prolactin. Milk ejection from the breast is stimulated by oxytocin. When an infant begins nursing, suckling on the breast typically triggers the milk ejection reflex.