Studies on Chromosomes 75 



the chromosome nucleolus and a number of the other bivalents. 

 Photo 22 shows the same condition. Photo 23, from the same cyst, 

 is slightly later, showing the two spheroidal ra-chromosomes wide 

 apart, the chromosome nucleolus, and several of the other chro- 

 mosomes. (The chromosomes nucleolus, perfectly recognizable 

 in the preparation, is in the photograph hardly distinguishable 

 from the other bivalents seen endwise.) Up to this point, which 

 shortly precedes the dissolution of the nuclear membrane, the 

 chromosome nucleolus is still immediately recognizable by its 

 deeper color (especially after safranin). There follows a brief 

 period in which this distinction disappears, but the chromosome 

 nucleolus is still recognizable by its asymmetrical form. That it 

 gives rise to the eccentric "accessory" is, I think, beyond doubt. 

 The evidence is demonstrative that it does not divide to form the 

 m-chromosomes, and that the latter arise from separate rods as 

 described. Gross appears to have seen these rods at a much 

 earlier period (cf. his Fig. 10) and correctly identifies them with 

 the spermatogonial m-chromosomes; but he believed them to give 

 rise to the "accessory." 



The relation of the chromosome nucleolus to the spermato- 

 gonial chromosomes cannot be determined in Syromastes with the 

 same degree of certainty as in Pyrrochoris (as described beyond) , 

 but the size relations leave hardly a doubt that Gross was right 

 in asserting its origin from two of the larger of these chromosomes. 

 The study of these relations is of importance because I believe 

 they justify the conclusion that the chromosome nucleolus, and 

 hence the "accessory," is nothing other than a pair of slightly 

 unequal idiochromosomes, which can readily be recognized in the 

 spermatogonial groups. 



Study of the spermatogonial groups in detail shows that twenty 

 of the chromosomes may be equally paired, while the remaining 

 two are slightly but distinctly unequal in size. These can always 

 be recognized as the smallest of the chromosomes except the 

 m-chromosome . Photos I and 2 show two groups in which this 

 clearly appears. These photographs are reproduced in Text Figs. 

 i<3, ib, with two others, c and d, the chromosomes in question 

 being designated as I and /'. 





