484 



THE EEPEODUCTIVE SYSTEM 



able for a study of the continuous process. From wall to lumen of 

 tubules can be seen, at successive levels, spermatogonia, primary sperma- 

 tocytes, secondary spermatocytes (prespermatids), spermatids, and sper- 

 matozoa attached to the tips of the sustentacular cells. Mitoses can 

 frequently be seen in spermatogonia, primary and secondary spermato- 



FIG. 429. SUCCESSIVE STAGES IN THE METAMORPHOSIS OF THE SPERMATID INTO 



THE SPERMATOZOON. 



(Schematic, from Bonnet, after Meves.) 



cytes. The primary spermatocytes are the largest of the germ cells; 

 the secondary spermatocytes are approximately half the size and lie 

 distal, while the spermatids are approximately one-fourth the size; the 

 spermatids may be at any stage of a continuous process of metamorpho- 

 sis involving nuclear, cytoplasmic, and general form changes. The 

 spermatogonia are of various sizes, frequently approximating that of 

 the primary spermatocytes, but always lie peripherally, on the basement 

 membrane between sustentacular cells. The above named marks con- 

 stitute the grosser criteria for identifying the several generations of 



