SPERM ATOGENE SIS OF THE RABBIT. 30! 



double the number, minus four, of the chromosomes of the 

 primary spermatocyte. 



Previous to the pachytene stage the cells have increased but 

 very little in size. Now they begin to grow rapidly, complete 

 fusion occurring between the conjugating threads, and all 

 gradually drift back to the center of the cell, there to spread 

 slowly throughout the nucleus. The threads enlarge, become 

 lighter in staining capacity, due perhaps to the greater volume 

 occupied by the pachytene thread, and finally form the large 

 spireme (Figs. 17, 20, 21). 



This stage of the primary spermatocytes, with the possible 

 exception of the synezesis stage, persists longer than any other 

 stage of the spermatogenesis. This conclusion is reached because 

 these stages of the primary spermatocytes can be found in prac- 

 tically any portion of the tubules. Numerous as they are, it is 

 extremely difficult to gain any knowledge of the processes in 

 progress in them. 



The accessory elements still retain their spherical form, usually 

 lying closely side by side, but at this time occasionally in different 

 portions of the cell (Figs. 17, 21). The chromatoid body enlarges 

 and seems to become more prominent. At the same time it 

 seems to acquire an activity in the cell although its function 

 could not be determined. 



4. REDUCTION DIVISION. 



The spireme of this stage in general appeared to be continuous, 

 although some of the smears made under considerable pressure 

 gave indications of segmentation (Fig. 23). The large spireme 

 now condenses and soon forms the individual chromosomes. 

 Up to this period of condensation the chromomeres stand out 

 very distinctly (Figs. 18, 19). These condense to form the large 

 mass of bivalent chromosomes which, because of their agglutina- 

 tion, usually stain so dense that the individual structure is 

 entirely lost. 



The X and Y elements, however, can nearly always be iden- 

 tified in these stages. As soon as the bivalent chromosomes are 

 formed, they migrate into the equatorial plate. The accessory 

 elements are nearly always a little to one side of this plate 



