GAMETOGENESIS 133 



multiplication of cells (the so-called spermatogonia) derived from 

 the germinal epithelium, in the testis, by ordinary cell-division. 

 Let us suppose the number of chromosomes found in the nuclei 

 of the somatic cells to be eight ; it will, of course, remain the 

 same so long as the character of the mitosis undergoes no 

 change, each chromosome splitting into two at every nuclear 

 division. Presently, however, we find the chromosomes arranged 

 in pairs instead of all appearing separately in the mitotic figure, 

 and the cells, which have increased considerably in size by the 

 absorption of nutriment, may now be termed primary spermato- 

 cytes. This pairing of the chromosomes (synapsis) marks the 

 onset of the "reducing division " ; a nuclear spindle is formed, the 

 paired chromosomes arrange themselves upon it, and the two 

 members of each pair separate and travel towards opposite poles. 

 Thus two new nuclei are formed each with only four chromosomes. 

 The reduction is now complete and the new generation of cells, 

 with reduced nuclei, may be termed secondary spermatocytes. 

 One more mitotic division takes place, this time involving the 

 splitting of each chromosome, so that there is no further reduc- 

 tion in their number, and giving rise to the minute spermatids, 

 each of which develops a long, vibratile, cytoplasmic tail and 

 forms a spermatozoon. Hence we see that each primary 

 spermatocyte gives rise to four spermatozoa, with reduced 

 nuclei containing half the number of chromosomes found in the 

 somatic cells. The essential features of the whole process are 

 represented diagrammatically in Fig. 65. 



The process of oogenesis takes place in essentially the same 

 manner ; the so-called oogonia, 1 derived from the germinal 

 epithelium of the ovary, multiply and give rise to oocytes. 

 Synapsis and reduction in the number of the chromosomes take 

 place as they do in spermatogenesis, but, owing doubtless to the 

 fact that it takes a comparatively large amount of cytoplasm to 

 form the body of an egg, we find that only one perfect ovum 

 arises from each primary oocyte, the other three forming the 

 " polar bodies." Owing to their minute size as compared with 

 the ovum itself, two of the polar bodies (Fig. 35, A, p.b.) appear 

 to be cast out of the latter as it undergoes maturation, while 

 the third is formed by division of the first ; it will be clear, 

 however, from a careful study of Figs. 65 and 68 that the 



1 This term is very unfortunately chosen and must not, of course, be confounded 

 with the same term as applied to the female organs of such plants as Fucus. 



