PROCESSES RELATED TO SEXUAL REPRODUCTION 225 



functional germ cells are differentiated at a later period. For 

 example, in some vertebrates the cells that actually produce the 

 germ cells may arise from the epithelial cells of the ovaries and 

 tests even in the adult organism. As the matter stands, it calls for 

 renewed investigation, although many instances of early differen- 

 tiation appear to be well established. In general, it may be said 

 that the germ cells are relativelj^ stable, for all their dependence 

 upon the body; and that in many instances they appear as a type 

 of cell that can be recognized in the early stages of the individual's 

 development. 



In the reproductive organs of the adult animal the relationships 

 of the germ cells are, of course, readily established. For example, 

 in the ovaries of most animals there are germ cells surrounded by 

 smaller cells which have a supporting and nutritive but not a 

 germinal function. Thus, the ovary of the frog, as seen in the fall 

 (Fig. 210, p. 401), shows cells of various sizes: the ova of the 

 coming spring, which appear as large cells; other cells of inter- 

 mediate size, representing approximately those of the next suc- 

 ceeding year; and man}' smaller cells, constituting the reserve 

 from which the eggs of subsequent j-ears will be differentiated. 

 In addition to these there are other cells that form a matrix of 

 tissue in which the ova are embedded. The ovary of a mammal 

 presents similar relationships, except that fewer eggs are produced 

 at each breeding season and hence the stock is relatively less 

 extensive. Nevertheless, there are within the human ovary 

 tens of thousands of potential genu cells from which mature eggs 

 might be developed. The testis presents a similar condition, 

 although the number of spermatozoa is so much greater than the 

 number of ova that the multiplication of the primordial germ cells 

 is accordingly increased. 



The foregoing explanations of the early appearance of the germ 

 cells and their relationships within the reproductive organs will 

 render more intelUgible the history of these cells. In the case of 

 the male germ cells, the zygote, or oosperm, from which an individ- 

 ual originates, is represented by the uppermost circle in Fig. 

 113. Within this are included outlines to represent chromo- 

 somes, which are taken as eight in this instance. The unshaded 

 chromosome, which represents the single sex-chromosome of the 

 male, need not be considered in the present discussion. From this 

 zygote there arise many cells by mitotic di^dsion, all with the same 



