MATURATION. 



25 



■filament — which winds around the axial filament of the middle piece. Mean- 

 while the axial filament has been growing in length and part of it projects be- 

 yond the limits of the cell. The cytoplasm remaining attached to the anterior 

 part of the filament surrounds it as the sheath of the middle piece. In Mam- 

 mals 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. 



Head 



Anterior end knob 

 Posterior end knob 



End ring 



HrL Anterior end knob 

 A? — i Posterior end knob 



Nucleus 



Cytoplasm 



L- Proximal centrosome 



Distal centrosome 



Fig. i 6. — Transformation of a spermatid into a spermatozoon (human). Schematic. 



Meves, Bonnet. 



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 of cytoplasm which covers the head is undoubtedly a remnant of 

 the cytoplasm of the spermatid. 



The developing spermatozoa lie with their heads directed toward the base- 

 ment membrane, and attached, probably for purposes of nutrition, to the free 

 ends of the Sertoli cells (Fig. 15). Their tails often extend out into the lumen 

 of the tubule. When fully developed they become detached from the Sertoli 

 cells and lie free in the lumen of the tubule. 



