162 EZRA ALLEN 



The exceedingly painstaking work of Regaud ('01) on this sub- 

 ject was not accessible to me until after I had come to the con- 

 clusion stated in the preceding paragraph. I have gone over my 

 material afresh, but see no reason for changing my original inter- 

 pretation. Since the question is one upon which I differ from 

 both Regaud and Duesberg, who have done the most upon this 

 subject among the more recent workers on the rat, I detail fully 

 below my evidence and arguments for my conclusion, drawing 

 chiefly upon the series of young rats. Regaud ('01) followed 

 the stages of growth in a series of young rats also, but he bases his 

 argument upon a study of many sections of adult tubules. 



The evidence which I have from the young series follows: 



1 . Type B are the first cells which differentiate from type A in 

 the young tubule after birth. 



2. Their first appearance is in very small numbers (fig. 3), but 

 quite general throughout the testis. 



3. Cell division is not common in the tubules which show type 

 B abundantly. In rat No. 150, ten days old, some tubules show 

 the outer layer of cells in very active division, but no cells of 

 type B are present. Other tubules show many cells of type B, 

 but no cell division. Occasionally a tubule shows cell division 

 and some cells of type B, but the latter are small and lie side by 

 side, indicating that they are daughter cells freshly produced 

 (fig. 5, B). In this last case all the inactive cells are ordinary cells 

 of type A, though some are manifestly preparing for division. 



4. Type B cells migrate from the outer edge to the inner por- 

 tion of the tubule. No cell division is seen in this portion of the 

 tubule (the inner portion) until the first spermatocytes are clearly 

 differentiated by their characteristic mitotic figures. 



5. The number of cells of type B usually becomes quite large 

 before these cells advance to the next well-marked stage (lepto- 

 tene), but the number of these leptotene cells never seems to 

 exceed the number which one would expect from the supply of 

 type B in tubules of the same size. 



Argument from the above evidence : The first two points have 

 no particular value alone, since the facte noted might be inter- 

 preted to indicate that the cells of type B were differentiated by 



