STUDIES ON CHROMOSOMES 75 



In N. hilaris the conditions differ only in that the two compo- 

 nents of the central chromosome are but slightly unequal; but in 

 the examination of at least two hundred of these divisions I have 

 never failed to detect the inequality. A series of side views are 

 shown in fig. 2 a-i, figs. 16-21, two of which show all the chromo- 

 somes. These figures illustrate practically all the variations 

 that have been seen in the idiochromosomes. The most charac- 

 teristic condition is that seen in 2 a, b, d, in which both idiochro- 

 mosomes (X and Y) are more or less elongated and united end to 

 end. Less often one of them assumes a more spheroidal form 

 (fig. 2 e, h, i, fig. 17). The size-difference, though always evident, 

 seems to vary slightly (perhaps because one or the other compo- 

 nent may be more or less compressed laterally), but is always dis- 

 tinctly greater than that now and then seen in other bivalents. 



Fig. 2 j shows a mid-anaphase 2 (cf. figs. 21-23) in which the 

 inequality would hardly be noticed without close study and the 

 comparison of other cases. Figs. 2 k and I are similar stages 

 showing all the chromosomes spread out in a series for the sake 

 of comparison. In both, the two idiochromosomes are easily 

 distinguishable, 3 and the larger is seen to be one of the three largest 

 chromosomes. Figs. 2 w-n, o-p, q-r and s-t are pairs of sister- 

 groups, in each case from the same spindle in anaphase. All of 

 these are selected from cases in which a distinct size-difference 

 appears between X and Y, but there are also many cases in which 

 this cannot be seen. Such a case was figured in fig. 4 e-f of my 

 first ' Study' the correctness of which is confirmed by re-examina- 

 tion of the original section. This condition is due simply to the 

 fact that the large idiochromosome is more elongated than the 

 small, so that the size-difference cannot be seen in polar view; 

 and for the same reason it is often not evident in polar views of 

 the metaphase. 



2 This and the two following figures are a little more enlarged than the others . 



3 Fig. 2 1 is the same group figured in fig. 4 d of my first 'Study,' carefully redrawn 

 and corrected. A comparison of the two drawings will show that in the latter a 

 distinct size-difference between X and Y is actually shown but is minimized by 

 the fact that the former is represented a trifle too small, the latter a little too 

 large. It is now also evident that they are connected by two connecting fibres 

 instead of by one. 



