138 EZRA ALLEN 



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The history of the chromosomes during the first spermatocyte changes 



This portion of the paper will be divided into two parts: 

 1) The early period of development, up to the stage when 

 the chromosomes are ready to assmne the characteristic tetrad 

 forms, or the diplotene stage, and, 2) the later period, covering 

 the changes occurring from the beginning of diakinesis through 

 the stages commonly known as prophases, and up to their meta- 

 phase and first maturation division. 



The rats which furnished the material for the early period were 

 aged, respectively, 7, 10, 11, 12, 13, 14, 15, 16, 20, 24, 29, and 35 

 days after birth, each from a different litter. This series enabled 

 me to identify positively each stage in the early development of 

 the first spermatocyte chromosomes. The results are recorded 

 in figures 2 to 15. 



For the study of the later stages, testes from two rats aged 37 

 and 50 days after birth, respectively, were the most serviceable. 

 Material of this age seems to fix better than that which is much 

 older, although in one case exceptionally good conditions were 

 found in a rat aged 180 days. Ripe spermatozoa are to be found 

 in the lumina of the tubules of 37-day rats, so that spermatogene- 

 sis is complete at that age. Other testes from older rats con- 

 tributed also, but to much less extent than the two first mentioned. 



1 . The early period. These stages are illustrated in the series of 

 figures in plate 2. The cross-sections of the tubules show the 

 changes which take place in the development of the spermato- 

 genic layers. Since these are all photographed at the same 

 magnification, they show the diameter changes incident to growth 

 as well as the arrangement of the cells. The more enlarged 

 figures show the cells in greater detail in their progressive devel- 

 opment. 



Preleptotene stages. The cells in figure 2, from a 7-day testis, 

 are apparently undifferentiated. The nuclei lie in a syncytium 

 which fills the tubule. For convenience I call these cells Type A. 

 Two of them are seen dividing {D.C., the left-hand one out of 

 focus). The larger, round cells in which the nuclear plasm is 

 less dense are preparing for division. These cells of type A 

 appear again clearly in the other figures. 



