THE REPRODUCTIVE ORGANS. 



245 



"basal plate," directly upon the basement-membrane, where the 

 edges of the basal plates are in contact, forming a sort of bed 

 with depressions in its upper surface, in which the spermatogonia 

 find lodgement. The cells of Sertoli possess a thick cytoplasmic 

 process, which extends toward the lumen of the tubule, and to 

 which those spermatids which are developing into spermatozoa 

 become attached. For this reason they are called sustentacular 

 cells. Their nuclei differ from those of the neighboring spermato- 

 gonia in being less rich in chromatin and 

 in possessing a single and prominent nu- 

 cleolus. 



The appearances of the various cells 

 enumerated depend upon the stage in 

 their activity which happens to be under 

 observation. The general course of de- 

 velopment, ending in the formation of 

 the spermatozoa, is as follows : the 

 spermatogonia, between the cells of Ser- 



Fig. 229. 



toli, multiply until quite a collection of 

 such cells is produced. Each division is 

 followed by a period of rest, during which 

 the chromatin increases in amount. When 

 the final stage of rest is at an end and the 

 cells have attained their maturity, they 

 constitute what are called the primary 

 spermatocytes. These now divide, each 

 forming two secondary spermatocytes, 

 which in turn divide, without an inter- 

 mediate distinct resting-stage, to form 

 two spermatids. Each primary spermato- 

 cyte, therefore, gives rise to four sperm- 

 atids. It is during the division of the 

 secondary spermatocytes that the reduc- 

 tion in chromatin, which was mentioned 

 above, takes place (Figs. 217-228). Each 

 spermatid receives, in addition to its por- 

 tion of chromatin, a single centrosome. 



The spermatozoon, then, is derived from a corpuscle, the spermatid, 

 which contains all the essential organs of a cell, differing from the gen- 

 eral cells of the body, the somatic cells, only in possessing half the 



Human spermatozoa. (Bohmand 

 Davidoff, after Retzius and 

 Jensen.) The left figure repre- 

 sents the side view and the 

 middle figure surface-view of 

 a spermatozoon, a, head (nu- 

 cleus) ; b, end-knob (centro- 

 some?); c, middle piece; d, 

 tail of flagella ; e, end-piece. 

 The thickness of d may be 

 owing to the presence of a 

 sheath surrounding the actual 

 flagella, which projects from 

 the sheath at e. 



