80 



COLLEGE ZOOLOGY 



and sperms; actually they are double-sexed. 

 In some species the eggs of an individual are 

 fertilized by sperms from the same individ- 

 ual; this is called self-fertilization. In other 

 species the eggs of one individual are fertil- 

 ized by the spermatozoa from another indi- 

 vidual; this is called cross-fertilization. The 

 earthworm is a common hermaphroditic 

 animal. 



Oviparous animals are those that lay eggs 

 which hatch outside the body of the mother; 

 for example, birds. 



Viviparous animals usually give birth to 

 young that develop from eggs within the 

 body of the mother and are nourished 

 from her blood stream; for example, mam- 

 mals. 



Ovoviviparous animals produce eggs that 

 hatch within the mother's body, but are not 

 nourished by the mother's blood stream 

 through a placenta; for example, certain 

 sharks and reptiles. 



Origin of mature eggs 

 and spermatozoa 



Oogenesis 



The primordial germ cells that give rise 

 to eggs or ova multiply by mitosis (see pp. 

 24-27). As a result, many oogonia are pro- 

 duced. The number of oogonia that arise 

 from each primordial germ cell may be defi- 

 nite as in some insects, or indefinite as in 

 most of the metazoans. As illustrated in 

 Fig. 404, p. 576, the oogonia grow in size 

 and are then called primary oocytes. In- 

 stead of dividing into two cells of equal size, 

 each oocyte undergoes an unequal division; 

 the larger daughter cell is known as a sec- 

 ondary oocyte, and the smaller daughter 

 cell, as the first polar body. The first polar 

 body may disintegrate, or divide into two 

 cells which eventually disintegrate. The sec- 

 ondary oocyte divides unequally, producing 

 a large cell, which we recognize as a mature 

 egg, and a small cell, called the second polar 

 body, which disintegrates. During the divi- 



sions of the primary and secondary oocytes, 

 the number of chromosomes is reduced to 

 one-half that in the oogonia. The signifi- 

 cance of this will be discussed in the chapter 

 on heredity. The end result of the divisions 

 of the oocytes is a mature egg, and the 

 process is referred to as maturation (mei- 

 osis). In many species eggs are laid before 

 the two maturation divisions occur; eggs 

 are considered immature until after the 

 polar bodies are formed. 



Spermatogenesis 



The stages (Fig. 404) in the origin of the 

 male sex cells, the spermatozoa, are very 

 similar to those of the eggs (Fig. 404). An 

 indefinite number of spermatogonia are pro- 

 duced by the primordial male germ cells. 

 These increase in size and then are called 

 primary spermatocytes. By the division of 

 each primar)' spermatocyte, two similar sec- 

 ondary spermatocytes are produced, and not 

 one functional and one nonfunctional cell 

 as when the primary oocyte divides. Like- 

 wise, when the secondary spermatocytes di- 

 vide, each produces two functional cells; 

 these are called spermatids. The spermatids 

 change without further division into sperms 

 (spermatozoa). During the maturation of 

 the male sex cells, the number of chromo- 

 somes is reduced to one-half the number in 

 the spermatogonia. 



Much scientific study has contributed to 

 our knowledge of the sperm and its function. 

 It is a highly specialized cell that can neither 

 grow nor divide; it consists essentially of a 

 condensed nucleus with means for locomo- 

 tion and penetration of the egg. It plays no 

 part in the physiology of the animal that pro- 

 duces it; its only function is the production 

 of a new individual. 



Fertilization 



The final result of the oocyte divisions is 

 one mature egg, and of the spermatocyte di- 

 visions, four sperms (Fig. 404). The mature 

 ovum now becomes the center of the inter- 



