168 EZRA ALLEN 



weighing 12.5 grams, very few tubules showed any stages beyond 

 type A. Some showed type B, while those having advanced as 

 far as the pachytene stage were very few indeed, and are to be 

 regarded as exceptional. 



Rat No. 149, 11 days old, weighing 10 grams, exhibits no 

 pachytene stages, but a considerable development of type B cell. 

 The 12-day rat, weighing 11.7 grams, showed very little develop- 

 ment of type B, these few cells being found in only a few tubules. 

 In this case the whole organ seemed to be retarded. The 13-day 

 rat, weighing 10.8 grams, shows type B cells in great abundance 

 throughout the organ, and in a few tubules a few cells in the 

 leptotene stage. The tissue of the 14-day rat, No. 146, weighing 

 23 grams, is much more advanced than the 13-day specimen. 

 Many of the cells are in the leptotene and a few have reached the 

 pachytene stage. Type B is in great abundance. No tubule 

 shows only type A. The whole organ exhibits the growth stages. 

 While cell division is perhaps not very vigorous, cells of type B 

 are intermingled with type A in some tubules, and in some cases 

 the cells of the latter type have been crowded inward by type B, 

 as shown in figure 5, so that type B occupies all the outer layer 

 of cells next to the limiting membrane. This condition indicates 

 that just previous to the death of the rat, cell division of type A 

 had been very active, but had ceased with the production of this 

 outer layer of daughter cells, which by the time the rat was 

 killed had advanced to type B stage. 



The 15-day rat, weighing 13.6 grams, shows about the same 

 degree of development as the 14-day specimen, except that the 

 cells in the more advanced stages are less numerous. Cell 

 division, however, is very abundant. Various parts of the testis 

 were examined, revealing a uniform condition. The 16-day 

 rat, weighing 22.5 grams, shows in general a slight advance over 

 the 15-day rat, the most developed cells being in the diplotene 

 condition, the next step in advance of growth over that reached 

 in the 15-day testis. 



From this stage on the development of the cells is slower. 

 Great complexity of cell types has been reached and the whole 

 organ is growing rapidly. The weight of the testes at age 37 days 



