THE GENERATIVE ORGANS. 191 



cleated cells ; and upon these lie two or three irreg- 

 ular layers of smaller nucleated cells with ill-defined 

 cell-bodies. Between the cells of the inner layers 

 a peculiar, irregular-branching net-work is seen, 

 which, by some recent observers, is believed to be 

 formed, for the most part, by branches of the outer 

 row of cells ; and it is supposed that it serves as a 

 loose framework in which the inner cells are sup- 

 ported. Others believe it to be only intercellular 

 cement-substance. The lumen of these tubules 

 may be filled with granular material or may contain 

 spermatozoa. 



In tubules which are in full functional activity, 

 the cavity is usually filled with more or less granular 

 material and mature and immature spermatozoa, and 

 on every side a multitude of clusters or bundles of 

 developing spermatozoa are seen, with the heads 

 imbedded among the cells of the inner layers, which 

 are similar to the lining cells above described, and 

 the tails stretching brush-like into the cavity of the 

 tube. 



According to some observers, the spermatozoa are 

 formed from certain of the cells of the inner layers, 

 which are called sp er mat ob lasts. The process of de- 

 velopment, according to this view, commences in the 

 nuclei of the spermatoblasts, which become con- 

 verted into the head, while the tail is an outgrowth 

 from the nucleus, or is produced by a transformation 

 of a portion of the cell-protoplasm. Others believe 



