116 



GENERAL EMBRYOLOGY 



cell the mature ovum, and another small cell, the second polar 

 body. Meanwhile the first polar body divides equally forming 

 two similar polar bodies. In some cases the division of the 

 first .polar body is suppressed. Thus each primary odcyte 

 typically gives rise, like the primary spermatocyte, to four cells, 

 but these are not all alike in form and size, although they are 

 fundamentally equivalent, i.e., homologous, to each other, and 

 to the four spermatids. Of these four cells, however, only one, 

 the ovum, is functional; the polar bodies degenerate without 

 functioning. The parallel events of spermatogenesis and 

 oogenesis are shown diagrammatically in Figs. 60-61. (See 

 also Figs. 76, 90, 94.) 



o'v 



FIG. 62. Longitudinal section through the ovary of the Copepod, Cantho- 

 camptus. From Wilson, "Cell," after Hacker, og, the youngest germ-cells or 

 oogonia (dividing at og. 2 ) ; a, upper part of the growth-zone; oc, oocyte^or growing 

 ovarian egg; ov, fully formed egg, with double chromatin-rods. 



All these processes may be going on in the ovary or testis at 

 the same time, occurring progressively from the basement 

 membrane of the germinal epithelium toward its free surface, 

 so that a section through such an epithelium shows practically 

 every step in the history of a single cell (Figs. 62, 68, 69). 

 Before considering in detail the nuclear changes involved in 

 the maturation processes we must consider the more important 

 facts concerning the history of the cytoplasmic parts during 

 this phase of the genesis of the germ cells. 



