Studies on Chromosomes 8 1 



some is nearly twice as large as any of the others, and is obviously 

 unpaired. I have examined a large number of spermatogonial 

 groups with great care with a view to the possibility that this chro- 

 mosome might in reality be double, but am thoroughly convinced 

 that such is not the case. This is unmistakably evident when this 

 chromosome has the form of a straight or only slightly curved rod 

 (Photos 24 to 28, Text Fig. 2, a to /), and these constitute the 

 great majority of observed cases. I have, however, found a few 

 cases where it has a very marked sigmoid curvature; two or three 

 of these give at first sight the appearance of two chromosomes in 

 contact (Photos 29, 30, 31 ; Text Fig. 2, /, k, I), Even here close 

 study shows that it is a single body; but such forms might readily 

 mislead an observer having a preconceived idea of the number to 

 be expected. 



That this is a single chromosome that is identical with the idio- 

 chromosome of the growth period and the maturation divisions 

 is placed beyond doubt by a study of the presynaptic stages, 

 which were not examined by either Henking or Gross, This 

 period is of such interest in Pyrrochoris as to merit a special study. 

 With only a single exception I know of no other form in which the 

 history of the idiochromosome and the succession of the stages can 

 be so completely and readily followed at this time. Throughout 

 this whole period, beginning with the telophases of the last sperma- 

 togonial division, the idiochromosome can be traced step by step 

 as a single body, and it is evidently identical with the large un- 

 paired spermatogonial chromosome. 



In the stages that immediately follow the last spermatogonial 

 telophase (Photos 32 and 33) the chromosomes still retain their 

 boundaries, though they show a looser texture, vaguer outlines 

 and diminished staining capacity (by which characters the post- 

 phases are readily distinguishable from the prophases). The large 

 chromosome (idiochromosome) is clearly distinguishable at this 

 time, both by its size and by its deeper color. In the stages that 

 immediately follow a remarkable contrast appears between this 

 chromosome and the others. The latter rapidly lose their visible 

 boundaries and their staining capacity, breaking up into a fine net- 

 like structure in which traces of a spireme-like arrangement may 



