174 THE REPRODUCTIVE SYSTEM. 



seminal ducts are conducting-channels or secrete fluid constit- 

 uents of the semen. 



The convoluted tubules are single or branched tubes, of 

 large diameter, coiled, bent, and twisted in intricate convolu- 

 tions, closely packed together so as to fill practically the entire 

 testicles. They are surrounded by well-marked basement- 

 membranes, and their coils are separated from one another by 

 the thin intertubular connective tissue. The tubules are lined 

 by several layers of epithelium, from which the spermatozoa 

 are generated. 



The deepest or parietal layer of epithelium consists of large 

 spheroidal spermatogenic cells (" spermatogonia ") resting on 

 the basement-membrane, among which are occasional long 

 sustentacular cells (" cells " or "columns of Sertoli") projecting 

 inward among the other cells toward the lumen. 



Upon these parietal cells rests a layer or two of large 

 spheroidal cells, mother-cells, derived from the " spermato- 

 gonia." Next internal to these are a greater number of 

 smaller spherical-cells ; these are called daughter-cells (also 

 called " spermatids " and " spermatoblasts "), and are derived 

 from the mother-cells. 



The daughter-cells elongate and develop into the sperma- 

 tozoa, (described below), which are located in clusters, their 

 heads (until they become separated) being attached to the 

 daughter-cells and their tails projecting into the lumen. The 

 lumen of the tubules contains, besides the spermatozoa, a 

 broken-down granular material. 



Spermatogenesis : The development of the successive layers 

 and generations of spermatogenic cells and the spermatozoa 

 takes place by karyokinesis. 



All parts of the seminiferous tract are not simultaneously 

 engaged in active cell-division, but the different stages of the 

 process are observable in different regions at the same time. 

 While karyokinesis and the production of spermatozoa are 

 actively going on in some places, in others the cells are 

 quiescent and spermatozoa absent. Some of the details of 

 the process of generation of the spermatozoa are still undeter- 

 mined or in dispute. It is supposed that during their 

 generation the number of chromosomes of the spermatozoa 



