20 TEXT-BOOK OF EMBRYOLOGY. 



cell. The nucleus passes to one end of the cell and becomes oval in shape. 

 Its chromatin becomes very compact and is finally lost in the homogeneous 

 chromatin mass which forms the greater part of the head of the spermatozoon. 

 Both centrosomes apparently take part in the formation of the middle piece. 

 The one lying nearer the center becomes disk-shaped and attaches itself to the 

 posterior surface of the head. The more peripheral centrosome also becomes 

 disk-shaped and from the side directed away from the head a long delicate 



Head 



Anterior end knob 

 Posterior end knob 



Head 



, Anterior end knob 

 Posterior end knob 

 - End ring 



Nucleus 



Cytoplasm 

 Proximal centrosome 



Distal centrosome 



Tail 



FIG. ii. Transformation of a spermatid into a spermatozoon (human). Schematic. 



Meves, Bonnet. 



thread grows out the axial filament. The central portion of the outer cen- 

 trosome next becomes detached and in Mammals forms a knob-like thickening 

 end knob at the central end of the axial filament. In Amphibians this part 

 of the outer centrosome appears to pass forward and to attach itself to the inner 

 centrosome. In both cases the rest of the outer centrosome in the shape of a 

 ring passes to the posterior limit of the cytoplasm. As the two parts of the 

 posterior centrosome separate, the cytoplasm between them becomes reduced 

 in amount, at the same time giving rise to a delicate spiral thread the spiral 



