128 Embryonic Development of Ovary and Testis of Mammals 



1. Seminiferous Tubules. — The seminiferous tubules of the 2.5 cm, 

 pig embryo are rather distinctly limited by a membrana propria, still 

 exterior to which is a thin layer of small connective tissue cells forming 

 a capsular investment. The cells of the tubules are of two general 

 classes — germinative cells and primitive sex cells, between which classes 

 are found all intermediate forms. The germinative cells are identical 

 with those classed above under groups 2 and 3, in fact the conditions 

 are not altered in this stage. All classes of cells are dividing by mitosis 

 while many of the germinative cells appear to be undergoing amitotic 

 division. 



Conditions in every regard similar to those outlined above, hold good 

 in the rabbit material, the corresponding period being about the 16th 

 day of embryonic life. 



Transition forms connecting the germinative cells with the sex cells 

 occur in the pig embryos between 2.5 cm. and 13 cm. length (see 

 Plate IV, Fig. 14), later stages showing no transition forms. The 

 rabbit material being more extensive, shows the condition of the cells 

 of the seminiferous tubules up to the period of sexual maturity. Inter- 

 mediate forms of cells are found to connect these two types in all stages 

 from the 16-day embryo up to the stage 8 days after birth inclusive. 

 Testes of the 40-day rabbit show absolutely no connecting links, the 

 two classes of cells being there found in their purity. The germinative 

 cells occur in a single layer with their bases attached to the membrana 

 propria, while the primitive sex cells — spermatogonia — lie in the more 

 axial portions of the seminiferous tubules. 



A striking feature of the seminiferous tubules is their tendency to 

 branch and anastomose (Plate IV, Fig. 12), such tendencies manifesting 

 themselves in the very earliest stages, when the sex cords are first laid 

 down. 



As has been previously shown, the rete tubules are pushed into the 

 anterior end of the sex gland. In the pig, their tips project into an 

 axial space left between the inner tips of the radially arranged seminif- 

 erous tubules, while in the rabbit, the mesentery is broader than in 

 the pig ; hence there is left a space at the base of the testis, for the occu- 

 pancy of the rete tubules. 



2. Rete Tubules. — ^In the testis of the pig, the rete tubules remain 

 within the hilum until a period between the 8.5 cm. and 10 cm. stages, 

 during which time they grow rapidly down the axial space, almost 

 reaching the distal end. It is at this period that the rete tubules begin 

 to acquire a lumen and also to send out branches^ — tubuli recti — to the 

 inner ends of the seminiferous tubules. As manv as four of these 



