THE REPRODUCTIVE SYSTEiM 



351 



line the lumen of the tubule and are the direct progenitors of the spermatozoa. 

 In the transformation of spermatocyte into spermatid an extremely important 

 change takes place in the nucleus. This consists in a reduction of its chromosomes 

 to one-half the number specific for the species (page 59). The transformation of 

 the spermatid into the spermatozoon differs somewhat in different animals and 

 the details of the processmust be regarded as not yet definitely determined. The 

 nucleus of the spermatid first becomes oval in shape, and its chromosomes be- 

 come condensed into a small homogeneous mass, which forms the head of the 

 spermatozoon. During their transformation into the heads of the spermatozoa, 



Head 



Anterior end knob 

 Posterior end knob 



Anterior end knob 

 Posterior end knob 



End ring 



Nucleus 



Cytoplasm 

 Proximal centrosome 



Distal centroEcne 



Fig. 245. — Transformation of a Spermatid into a Spermatozoon (human). Schematic. 



(Meves, Bonnet.) 



the nuclei of the spermatids arrange themselves in tufts against the inner ends 

 of the cells of SertoH. This compound structure, consisting of a Sertoli cell 

 and of a group of developing spermatozoa attached to its central end, is known as 

 a spermatoblast (Fig. 238). The body or middle piece of the spermatozoon is 

 described by most investigators as derived from the centrosome, while the tail 

 is a derivative of the cytoplasm. 



The details of the transformation of the spermatid into the spermatozoon are 

 illustrated in Fig. 245. The centrosome either divides completely, forming two 



