THE MALE REPRODUCTIVE ORGANS. 



2O9 



FIG. 251. 





', 



exists ; inside the latter lies the lining of epithelial cells. The pre- 

 cise character of the cells within the tubule depends upon the 

 condition of functional 

 activity of the organ ; the 

 notable differences distin- 

 guishing the elements with- 

 in the resting gland from 

 those found in the active 

 organ depend largely upon 

 the infrequency within the 

 former, and the almost uni- 

 versal presence within the 

 latter, of cells actively en- 

 gaged in karyomitotic divis- 

 ion. The different tubules, 

 however, exhibit great vari- 

 ation in the exact stage 

 of these changes, adjacent 

 canals, and, in fact, parts of 

 the same one, often present- 

 ing the extremes of the 

 cycle side by side. 



Next the basemen t- 

 membrane of the seminiferous tubule lies a layer of low cuboidal 

 nucleated parietal cells ; this peripheral zone contains cells of 

 two kinds : (i) the sustentacular cells, or Sertoli's columns, 

 which take no part in the formation 

 of the generative elements, and (2) 

 the spermatogenic cells, which pro- 

 duce elements intimately related to the 

 development of the seminal filaments. 

 Inside the outer, peripheral layer, in 

 functionally active organs, an irregular 

 second zone contains many elements with 

 large transparent nuclei and chromatin 

 figures, indicating the progress of cell- 

 division ; these are the mother-cells, 

 the derivatives of the spermatogenic cells 

 of the outer zone, and, in turn, the parents 

 of a numerous progeny of smaller daugh- 

 ter-cells. The nuclei of the latter con- 

 stitute the spermatoblasts, since it is 



from them that the spermatozoa are directly derived. The inner 

 .zone of the tubule is frequently occupied by fan-shaped groups of 



14 



Transverse section of seminiferous tubule from human 

 testicle : a, membrana propria ; b, zone of parietal cells ; 

 c, mother-cells undergoing division ; d, daughter-cells, or 

 spermatoblasts ; e, partially-developed spermatozoa ; f, 

 surrounding intertubular connective tissue. 



FIG. 252. 



Section of testicle of dog, including 

 part of seminiferous tubule : a, zone of 

 parietal cells containing sustentacular 

 elements (f); 6, mother - cells ; c, 

 daughter-cells ; d, free nuclei of sper- 

 matoblasts and developing sperma- 

 tozoa. 



