14 



TEXT-BOOK OF EMBRYOLOGY. 



in shape. Its chromatin becomes very compact and finally condensed 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 bead. The more peripheral 

 centrosome also becomes disk-shaped and from the side directed away from 

 the head a long delicate thread grows out the axial filament. The central 



Head 



Anterior end knob 

 Posterior end knob 



Head 



. Anterior end knob 

 Posterior end knob 

 " End ring 



Tail 



Nucleus 



Cytoplasm 

 Proximal centrosome 



Distal centrosome 



FIG. 8. Transformation of a spermatid into a spermatozoon (human). 



Meves, Bonnet. 



Schematic. 



portion of the outer centrosome 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 tq 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 giv- 

 ing rise to a delicate spiral thread the spiral filament which winds around 



