MATURATION. 



15 



the axial filament of the middle piece. Meanwhile the axial filament has 

 been growing in length and part of it projects beyond the limit of the cell. 

 The cytoplasm remaining attached to the anterior part of the filament sur- 

 rounds it as the sheath of the middle piece. In mammals there appears to be 

 more cytoplasm than is needed for the formation of the sheath of the middle 

 piece, and a large part of it degenerates and is cast aside. The sheath which 

 surrounds the main part of the axial filament appears in some cases at any 

 rate to develop from the filament itself. The galea capitis or delicate film 



XY 



.XY2 



B C 



FIG. 9. Three stages in spermatogenesis in man (negro). Wieman. 



In a is shown a nucleus of a primary spermatocyte during the growth period; p, plasmosome; x 

 and y, accessory chromosomes. In b is shown the metaphase in a primary spermatocyte 

 in which there are 1 2 bivalent chromosomes that have resulted from synapsis of the 24 in 

 the spermatogonium, the x and y uniting with each other. In c is shown a later stage of 

 spermatocyte division in which the xy pair has divided longitudinally, the daughter chro- 

 mosomes passing toward the poles of the spindle ahead of the main group. 



of cytoplasm which covers the head is also a derivative of the cyto- 

 plasm of the spermatid. 



The developing spermatozoa lie with their heads directed toward the 

 basement membrane, and attached, probably for purposes of nutrition, to 

 the free ends of the Sertoli cells (Fig. 6) . Their tails often extend out into 



