SPERMATOGENESIS IN THE ALBINO RAT 163 



growth changes of type A and might divide to produce the sper- 

 matocytes. But the conditions noted under items 3, 4, and 5 

 would not be found were cells of type B spermatogonia, for in that 

 case, 1) cell division would be active in the groups of large undi- 

 vided cells of that type; 2) these dividing cells would be found in 

 the deeper layers as well as the outer, and, 3) the leptotene cells 

 would be twice as numerous as the cells of type B. 



The evidence from the adult tissue corroborates this con- 

 clusion, because no cell division occurs there in the layer of cells 

 next the cell membrane after the appearance of type B. Sperma- 

 togonial division occurs only once in the wave (except sporadic- 

 ally in the case of a cell or two), 'and that is at the time of the 

 division of the second spermatocytes and the early growth period 

 of the spermatids. The only cells of type B present then are 

 few and small, and evidently to be interpreted as daughter cells 

 as in figure 5. As the wave progresses, these cells enlarge with- 

 out division and are crowded into the next inner layer where they 

 pass into the leptotene stage. 



Regaud ('01) recognized cells similar to his spermatogonia 

 croutelleux as the early stages of first spermatocytes, and states 

 that the two kinds of cells are likely to be confused. It would 

 seem that his difficulty lay in not recognizing that when cells of 

 type A prepare for division, part of the chromatin becomes at 

 first a flaky, elongated woolly mass which later forms part 

 of the chromosomes, while the remainder of the chromatin 

 gathers in small masses underneath the nuclear membrane and 

 differentiates its share of the chromosomes. His fixation was by 

 potassium bichromate and acetic acid, a fixative which I found 

 produces much distortion. I note considerable difference in the 

 appearance of type B cells according to whether Fleixnning or 

 'B-15' is used as fixing fluid. Flemming's fluid produces chro- 

 matin masses with hard outlines; 'B-15' with softer, more woolly 

 outlines, yet clear and distinct from each other. 



The interpretation of type B cells as the earliest stages of first 

 spermatocytes rather than as a second kind of spermatogonia 

 which must divide to produce spermatocytes, is in agreement 

 with well-known insect material, especially well described by 

 Wenrich ('16) in Phrynotettix magnus. 



