STUDIES ON CHROMOSOMES a5 
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-7, 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, z, 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 7 shows a mid-anaphase? (ef. figs. 21-23) in which the 
inequality would hardly be noticed without close study and the 
comparison of other cases. Figs. 2 & and / 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,’ and the larger is seen to be one of the three largest 
chromosomes. Figs. 2 m—n, o-p, g-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. 21 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. 
