76 bulletin: museum of comparative zoology. 



C. The Individuality of the Chromosomes. 

 a. The selected Chromosome Pairs. 



The method adopted in the study of the subject of synapsis — that 

 of following the history of individual chromosome-pairs — has natu- 

 rally led to a consideration of the subject of the individuality of the 

 chromosomes, that is, their persistence as morphological entities 

 tlirough all the stages of nuclear activity. I have already attempted 

 to demonstrate that each of the chromosomes of the last sperma- 

 togonial division gives rise to a single leptotene thread and that these 

 single threads conjugate two-by-two in the zygotene stage. It will 

 be more convincing, however, if we can follow some particular chromo- 

 some-pair through these difficult stages. 



1. Chromosome-pair A. — As the chief characteristic by which the 

 chromosome-pair A could be recognized in the pachytene and later 

 stages of the first spermatocytes, I have already described its great 

 density and staining capacity. If there is a persistence of individual 

 chromosomes from the spermatogonia to the spermatocytes, we should 

 expect to find in the former a pair of chromosomes exhibiting th( same 

 peculiarities that the pair did in the later generation. Such a pair 

 can, indeed, be found in the telophases not only of the last spermato- 

 gonia! division but of the earlier spermatogonia as well. Figures 66 

 and 67 (Plate 6) show such pairs of chromosomes more deeply stained 

 than their fellows. Figure 66 shows one of the earlier generations of 

 spermatogonia, as is indicated by the vesicular condition of the ac- 

 cessory chromosome, and figure 67 represents a telophase of the last 

 spermatogonial division, as is shown by the condensed accessory at 

 this stage. 



It is difficult to follow all the changes that these chromosomes 

 undergo in their transformation into pachytene tlu'eads, but I believe 

 that most of the stages are represented in the series of drawings, 

 figures 67-78. Figure 67 corresponds to a stage midway between 

 those shown in figures 21 and 23 (Plate 2). Figure 68 is of a stage 

 corresponding very closely to that in figure 25 (Plate 3). In figures 

 68 and 25, two bands or "blocs" of chromatin can be seen which are 

 more deeply stained than the other chromatin-blocs. The accessory 

 chromosome is distinguishable by its characteristic density and its 

 position at the periphery of the nucleus. The polar granules are also 

 distinguishable. The chromatin in these darker blocs {A in both 



