INTERSTITIAL CELLS OF URODELE TESTIS 235 



from a man of eighty-four years. The occurrence of interstitial 

 cell mitoses in numbers in any mammal, then, seems limited to 

 its embryonic stages or early postnatal life, as compared with the 

 periodic (annual) multiplication of these cells in the urodeles. 

 It is suggested that in the testes of young mammals a cessation 

 of growth of the seminiferous tubules, or possibly degenerations 

 occurring within them, must furnish conditions favorable to 

 interstitial cell differentiation and multiplication similar to those 

 in the urodele testis at the close of its spermtogenetic cycle. 

 Allen, indeed, has stated that the interstitial cells in embryos 

 are ''formed contemporaneously with the appearance of fatty 

 degeneration in both peritoneum and seminiferous tubules;" 

 Whitehead, however, believes that they appear before any such 

 degeneration occurs. The latter, in describing in pig embryos 

 the development of interstitial cells from intertubular tissue of 

 mesenchymal structure, states that mitoses are seen in the early 

 stages of their differentiation. From a similar interlobular 

 tissue in the urodeles — a tissue with a syncytial arrangement 

 and scant cytoplasm, typically a mesenchymal structure — ^the 

 interstitial cells of urodeles differentiate, with numerous mitoses 

 marking the earlier stages of their transformation. The close 

 time relations of multiplication to differentiation from a connec- 

 tive-tissue type of cell is the same in the embryonic mammalian 

 testis as in the testis of the urodele, and it seems not unreasonable 

 to suspect the operation of fundamentally similar metabolic 

 conditions in both at the time these occur. As noted above, 

 Allen and Whitehead disagree as to what these conditions may 

 be in the embryonic testis. Whitehead reaching the conclusion 

 that ''the hypothesis which attributes the growth of Leydig's 

 cells to fatty degeneration in these situations is incorrect;" in 

 the urodele testis the favoring conditions are clearly those 

 resulting from the beginning of regressive changes in the lobules 

 at the close of the sex cycle. 



c. Condition in late fall and winter months. As the spermatozoa 

 mature first in the caudal portion of the testis, and first leave that 

 portion, it is there that the differentiation and mitoses of the 

 interstitial cells begin, as has already been stated. As the wave 



