Bennet Mills Allen 129 



tiibiili recti were seen to arise from a single rete tubule, being apparently 

 called forth wherever needed. A distinction between the tubuli recti and 

 the seminiferous tubules can be readily drawn from several criteria. The 

 chief difference lies in the greater diameter, lack of lumen, and far greater 

 number of sex cells of the seminiferous tubules as contrasted with the 

 fact that the rete tubules are narrower by half, possess a lumen, and 

 contain very few primitive sex cells (Plate V, Fig. 18). The two struc- 

 tures resemble one another in the character of their component cells, the 

 germinative cells of the seminiferous tubules being practically identical 

 with the epithelial cells of the rete tubules, and the primitive sex cells 

 of both structures showing an exact correspondence. This homology is 

 also seen in the fact that both structures are limited by membrana pro- 

 pria and capsular connective tissue investments formed in the same man- 

 ner in each. 



This process of the extension of the rete tubules takes place in the 

 rabbit 3 days after birth. Later, the seminiferous tubules grow about 

 the eccentrically placed mass of rete tubules in such a manner as to en- 

 close it. Eepeated anastomosis of the rete tubules results in the union 

 of their lumina to form a large cavernous, irregular space imperfectly 

 divided by the walls of the component tubules. The nuclei of the rete 

 cells still have the general characters of those belonging to the germi- 

 native cells of the seminiferous tubules; but are far more elongated by 

 lateral compression. Primitive sex cells are found in the rete tubules 

 of the rabbit 24 days after birth, but are not present in those of the 140- 

 day rabbit; hence it is safe to conclude that the sex cells of the rete tes- 

 tis are not functional. 



3. Interstitial Cells. — In the pig, the interstitial cells are found to 

 multiply by mitosis from the time of their first appearance up to the 

 stage of the 7.5 cm. embryo, and in the rabbit testis as late as 8 days 

 after birth. There may be new interstitial cells formed between the 

 period of their first appearance and sexual maturity, but this seems 

 highly improbable, no evidences of such having been seen. They begin 

 to degenerate in the 15 cm. pig embryo, and in the rabbit 24 days after 

 birth. This process of degeneration first manifests itself by a shrinkage 

 of the cytoplasm. In the process of development of these interstititial 

 cells, their cytoplasm becomes filled with fat globules that have a ten- 

 dency to run together (Plate IV, Fig. 13). At the same time, the 

 centrosphere becomes clearer and more sharply defined from the sur- 

 rounding cytoplasm. Plato, 97, does not represent the centrospheres in 

 his figures of the interstitial cells of the cat, rabbit, steer, horse and 

 other forms studied bv him. 



