140 EZRA ALLEN 



Zygotene stage. Following these stages just described, synapsis 

 occurs. Again the densely packed spireme makes observation 

 difficult. With care one finds the condition shown in figure 15, 

 indicating para-synapsis, union occurring at one end first. A 

 very slight polar aggregation may occur during this period, but 

 no synizesis has been observed with any method of fixation. 



Pachytene stage. The completed process of synapsis gives us 

 the pachytene condition, (figs. 5, 8, 14, and fig. 1, P.c). This 

 stage is recognized by the greater spaces between the spireme 

 threads and, in favorable places, by the double nature of the 

 threads. The union is extremely intimate, so that this double 

 nature is seldom visible except in the early stages. It is in this 

 stage that the cell first shows complete differentiation of the 

 cytosome. In adult material the syncytium has ceased to exist 

 and the spermatogonia are consequently individualized. In 

 the young material this differentiation of the cell is always pre- 

 ceded or accompanied by the appearance of the lumen in the 

 tubule, as shown in figures 5, 6, 7, and 8. The lumen in the 

 series studied appeared first in the testis of the 14-day rat, not 

 at all in the younger testes. 



Conclusions. It appears that the first spermatocyte cells be- 

 gin their differentiation with the organization of the nuclei 

 in the daughter cells of the last division of an undifferentiated 

 type of cell (Type A), which is the only type then present in the 

 tubule. This process begins between the age of 7 and 10 days 

 after birth. The pachytene stage is reached by the age of 14 

 days. For further considerations see the discussion, p. 160. 



2. The later period. As stated on page 138, the material for 

 this part of the study is mostly from rats aged 37 and 50 days. 

 Older material was compared with it and found essentially simi- 

 lar though not always so workable. The material younger than 

 37 days helped in understanding the first changes which occur in 

 early diakinesis. 



The changes from the pachytene condition to the formation of 

 tetrads and the organization of the different forms of spermatocyte 

 chromosomes are difficult to follow for each chromosome. How- 

 ever, the various stages in their development are readily com- 



