122 GENERAL CONCEPTS 



cytoplasm and yolk while the other, the first polocyte or polar body, is 

 essentially a bare nucleus. 



The secondary oocyte divides by the second meiotic division, again 

 with an unequal division of cytoplasm, to yield a large ootid, with 

 essentially all of the yolk and cytoplasm, and a small second polocyte. 

 (The first polocyte may divide at about the same time into two addi- 

 tional polocytes.) The ootid undergoes further changes (but no cell 

 division) and becomes a mature ovum (egg). The polocytes disintegrate 

 and disappear, so that each primary oocyte forms a single ovum, in con- 

 trast to the four sperm derived from each primary spermatocyte. The 

 formation of the polocytes is a device to enable the maturing egg to get 

 rid of its excess chromosomes, and the unequal division of the cytoplasm 

 insures the mature egg enough cytoplasm and yolk to survive and de- 

 velop if it is fertilized. 



The union of a haploid set of chromosomes from the sperm with 

 another haploid set from the egg during fertilization reestablishes the 

 diploid chromosome number. The fertilized egg, and all the body cells 

 which develop from it, have the diploid number, two of each kind. 

 Each individual gets half of his chromosomes (and half of his genes) 

 from his father and half from his mother. Because of the nature of gene 

 interaction, the offspring may resemble one parent much more than 

 the other, but the two parents make equal contributions to its inheri- 

 tance. 



35. Reproductive Systems 



In some of the simpler invertebrates, such as the coelenterates, the 

 testes and ovaries are the only sex structures present, and eggs and sperm 

 are released directly from the gonads into the surrounding water. Most 

 animals, however, have a system of ducts and glands which serve to 

 carry gametes from the gonad to the exterior of the body and to pro- 

 tect and nourish them during the process. 



Many of the lower animals are hermaphroditic; both ovaries and 

 testes are present in the same individual and it produces both eggs and 

 sperm. Some hermaphroditic animals, the parasitic tapeworms, for ex- 

 ample, are capable of self-fertilization. Since a particular host animal 

 may be infected with but one parasite, hermaphroditism is an im- 

 portant adaptation for the survival of the parasitic species. Most her- 

 maphrodites, however, do not reproduce by self-fertilization; in the 

 earthworm, for example, two animals copulate and each inseminates 

 the other. In certain other species, e.g., the oyster, self-fertilization is 

 impossible because the testes and ovaries produce gametes at different 

 times. 



The reproductive systems of different species have a fundamentally 

 similar plan, but many variations on the theme are evident. The gonads 

 and their ducts may be single, paired or multiple, perhaps present in 

 several segments of the body. 



The male reproductive system typically comprises the testes, vasa 

 efferentia and vas deferens. Sperm are produced in the coiled seminifer- 



