204 



THE ESSENTIALS OF HISTOLOGY. 



Structure of the tubules. The seminiferous tubules are formed of a 

 thick basement-membrane, and contain several layers of epithelium- 

 cells. Of these layers, the one next the basement-membrane is a 

 stratum of clear cubical cells (lining epithelium, fig. 241, a), the nuclei 

 of which, for the most part, exhibit the irregular network which is 

 characteristic of the resting condition, but in certain tubules they 

 exhibit indications of division. Here and there these epithelium-cells 

 appear enlarged, and project between the more internal layers, being 

 connected with groups of developing spermatozoa. These enlarged 

 cells may be termed sustentacular cells (fig, 241, a'). 1 



FIG. 238. SECTION OF PARTS OF THREE SEMINIFEROUS TUBULES OF THE RAT. 



a, with the spermatozoa least advanced in development ; 6, more advanced ; c, containing 

 fully developed spermatozoa. Between the tubules are seen strands of interstitial-cells 

 with blood-vessels and lymph-spaces. 



Next to this epithelium is seen a zone of larger cells (spermatogenic 

 cells, fig. 241, 6), the nuclei of which have the skein-like aspect which is 

 typical of commencing division ; these cells may be two, three, or more 

 deep (as in a, fig. 238). Next to them, and most internal, is to be seen 

 in some tubules (b and c) a large number of small protoplasmic cells 

 with simple circular nuclei (spermatoblasts, fig. 241, c). In other tubules 

 these cells are elongated, and the nucleus is at one end, and in others 

 again these elongated cells are converted into evident spermatozoa, 

 which lie in groups with their heads projecting between the deeper 

 cells and connected with one of the enlarged cells of the lining epi- 



1 These are the ' spermatoblasts ' of some authors a name given to them on 

 the erroneous supposition that they directly produce the spermatozoa. The term 

 ' spermatoblast ' is more applicable to the small cells of the third layer or zone, 

 and will be so applied here. 



