REPRODUCTION. 



comes the above-mentioned germinal epithelium of Waldeyer, because 

 it is these cells which, by their down-growth into the subjacent 

 layers, gradually become transformed, first, into indifferent sexual 

 cells, and, ultimately, into spermatozoa or ova, as the case may be. 

 Leaving out the detailed account of the development of the testicle and 

 ovary, to be found in text-books on embryology, we have to consider 

 the formation of the spermatozoa and ova as it takes place in a fully 

 developed ovary or testicle. In the seminiferous tubules of the testi- 



Fig. 381. Diagram Showing Development of Spermatozoa in a Seminal 

 Tubule. ( McMuERiCH. ) 



1, Spermatozoon. 2, Spermatid. 3, Secondary spermatocyte. 4, Primary sper- 

 matocyte. 5, Spermatogone. 6, Supporting cell. 



cle we find five physiologically different kinds of cells. Covering 

 the inner surface of the basement membrane of the tubule we find 

 the so-called layer of parietal cells, consisting of two kinds of cells, 

 of a different physiological character: (1) the sustentacular, and (2) 

 the spermatogenic cells. Both kinds undergo karyokinetic multipli- 

 cation, but the fate of their offspring is different. Each offspring 

 of a sustentacular cell, after attaining its full size, is not only mor- 

 phologically, but also physiologically, fully equivalent to the parent 

 cell, as it is ready to serve its ultimate definite purpose supporting 

 and generally uniting with other cells of its kind, it forms stronger 



