MATURATION. 21 



ovum and forms the first polar body (Fig. 12), thus leaving within the ovum 

 —secondary oocyte — twelve parts or pieces. Closely following the formation 

 of the first polar body, the twelve remaining pieces divide again transversely 

 and one-half of each piece passes out into the second polar body (Fig. 13). 

 Within the ovum — now the mature ovum — there still remain twelve pieces of 

 chromatin or twelve chromosomes — one-half the somatic number. As a 

 matter of fact, Sobotta, who studied the process in the mouse, observed in 

 the majority of cases only one polar body (Fig. 13, A) . Later investigators have 

 suggested that the second polar body also was probably formed but at such a 

 time or in such a plane that Sobotta had failed to see it. If only one polar 

 body is formed, the reduction is atypical ; but if, as suggested later, two polar 

 bodies are formed, it would make the process typical. In either case the result 

 is the same so far as the number of chromosomes is concerned, but in the 

 former case the bulk of chromatin is reduced only to one-half, in the latter 

 case it is reduced to one-fourth the original amount. 



SPERMATOGENESIS. 



The maturation of the male sex cells in the majority of forms is perhaps 

 more difficult of demonstration than the maturation of the female sex cells, 

 both on account of the extreme minuteness of the former and on account of the 

 fact that it is necessary to consider all the generations of cells from the mature 

 spermatozoa back to the spermatogonia. In some forms the reduction of 

 chromosomes has been demonstrated, and it is reasonable to assume that it 

 occurs in all forms. In Ascaris, for example, the reduction has been clearly 

 shown to occur, as in the case of the ovum, through tetrad formation (Fig. 14). 

 In the higher forms, and especially in Mammals, the process has not been 

 observed with such accuracy; but it is possible in a general way to trace the 

 changes through which the cells pass to arrive at the stage of mature sperma- 

 tozoa. 



In the mammalian testis (Fig. 15, A) the stratified epithelium of the con- 

 voluted seminiferous tubule consists of two distinct kinds of cells : (1) the 

 so-called supporting cells or cells of Sertoli, and (2) the different generations of 

 male sex cells or spermatogenic cells. In a portion of a tubule where active 

 formation of spermatozoa is not going on, the cells are arranged as follows. 

 Upon the basement membrane is a single layer of small oval or cuboidal 

 cells with nuclei rich in chromatin. These are known as spermatogonia. 

 Internal to these are one or two layers of larger round or oval cells with large, 

 vesicular, densely staining nuclei, the spermatocytes. Between these and the 

 lumen are several layers of small, oval cells whose nuclei contain closely-packed 

 chromatin granules. These are the spermatids. Usually the lumen of the 

 tubule contains a number of mature spermatozoa, which may lie either free in the 



