Bennet Mills Allen 123 



panying outward movement of the peritoneum results in a thickening 

 of the genital ridge in the sex gland region. This becomes more and 

 more pronounced until the sex gland appears hemispherical in trans- 

 verse section, later appearing as a disc attached to the mesonephros 

 by a relatively slender bridge — the mesentery. It gives the impression 

 of having become constricted from the surface of the mesonephros. This 

 appearance is, however, delusive, as the mesentery is in reality slightly 

 broader than was the base of the rudimentary sex gland. 



The cells composing the indifferent sex gland (Plates III and IV, 

 Figs. 9 and 11) may be classed under three heads: (1) Primitive sex 

 cells; (2) syncytial cells with small nuclei; (3) syncytial cells with 

 nuclei of various sizes. 



The primitive sex cells, already described, are found chiefly in the 

 sex cords, although they occur sparingly in the connective tissue of the 

 mesentery. They divide infrequently by mitosis throughout these early 

 stages. 



The nuclei of cells of class (2) stain deeply and are often attenuated. 

 They form the albuginea and occur in the stroma, sex cords and, to a 

 limited extent, in the peritoneum. 



Cells of the third class occur together with those of the second class, 

 which they resemble in that they are without definite boundaries. Their 

 nuclei are larger and usually stain less deeply, showing all gradations 

 between those of the primitive sex cells and the small, deeply-staining 

 syncytial cells of class ( 2 ) . It is almost certain that both forms originate 

 from these cells of intermediate character of which the peritoneum is 

 almost exclusively formed. 



In the basal portions of the sex cords at the time when the latter 

 are being separated from the peritoneum, there is a direct transition 

 of the nuclei of class (3) into connective tissue nuclei of class (2), which 

 forms the mesentery and the albuginea (Plate III, Fig. 9). Certain 

 small nuclei of the sex cords resemble tliese connective tissue nuclei in 

 a most striking manner and probably arise by a similar process of differ- 

 entiation. These last named belong to the germinative cells of the 

 seminiferous tubules and to the follicular cells of the medullary cords. 



The albuginea and mesentery of the sex glands are derived from the 

 peritoneum of regions immediately dorsal and ventral to the sex gland 

 (Plate II, Fig. 7). The cells of these mesentery rudiments proliferate 

 rapidly throughout the early stages, causing a rapid growth of the mesen- 

 tery. Here and there primitive sex cells are formed in these regions 

 and are carried down into the mesentery with the connective tissue in 

 the midst of which they lie. 



