THE MALE REPRODUCTIVE ORGANS. 



211 



Critical study convinces that the nucleus, together with the cen- 

 trosome of the spermatoblast, is the chief factor in the formation 

 of the spermatozoon. Without attempting a detailed account of 

 the complicated cycle, these changes may be briefly stated to con- 

 sist in the increase and accumulation of the nuclear chromatin in 

 a manner resulting in the differentiation of the nucleus into two 

 zones, an outer, which stains deeply and is rich in chromatin, and 

 an inner, which appears clear and is devoid of chromatin. Coinci- 

 dently with these changes the nucleus escapes from the proto- 

 plasm of the daughter-cell to take up 

 its position, in company with other 

 free nuclei, within the granular re- 

 mains of the extruded cell-proto- 

 plasm. Subsequently the chromatin 

 becomes especially condensed and ac- 

 cumulated at the inner border of the 

 darker outer half of the nucleus ; 

 from this zone of chromatin a deli- 



FIG. 256. 



FIG. 255. 



Section of testicle of musk-rat, dis- 

 playing still later stage of spermato- 

 genesis : spermatozoa are now well 

 advanced and form radially-arranged 

 masses (<f) ; other letters as in pre- 

 ceding figures. 



Section of human testicle, including parts 

 of three tubules (t) and intervening connective 

 tissue ; within the latter lies a group of inter- 

 stitial cells (i). 



cate projection or spine grows into, and, later, through, the inner 

 clear half of the nucleus ; this outgrowth is the first indication of 

 the future tail of the spermatic element. As the result of the local- 

 ization of the chromatin within the central part of the nucleus, the 

 latter now exhibits three zones : an outer clear cap at the fore- 

 pole, a narrow middle zone filled with chromatin, from which 

 the developing tail-fibre extends, and an inner clear area which 

 reaches as far as the hind-pole and is pierced by the tail. In these 

 three nuclear zones the divisions of the future mature spermatozoon 

 are indicated : the outer clear cap becomes the homogeneous head, 

 the middle chromatin band produces the tail and the middle-piece, 



