478 Chromosomes of Anasa Tristis and Anax Junius 
tropic. At about the stage of the anaphase represented in C, this in- 
equality in the number of chromosomes composing the daughter plates 
has been determined in several well marked cases where the groups have 
been observed in polar view at different focuses on one and the same 
spindle. # and F are such anaphase groups drawn from the same section, 
F including the heterotropic which is seen at h, F lacking this chromo- 
some. In each group the m-chromosome and the macro-chromosome are 
readily picked out, while the two groups, if added together, would exactly 
reproduce the conditions seen in the spermatogonial plate. 
It has been said that the heterotropic chromosome usually passes in 
advance of the others during the anaphase of the second division, as in 
the majority of cases, where this stage is observed in side view, such a 
condition is quite evident. But, strangely enough, a few cells are always 
encountered, often in the same cyst with the others, which show the heter- 
otropic lagging behind instead of preceding the other chromosomes, as 
in the case of Anasa. Of course it cannot be determined with certainty 
that the lagging one is the same chromosome as the precocious heter- 
otropic of the commoner cases, but there can be little doubt that the two 
are identical. This conclusion is rendered very probable from their simi- 
larity in size and also from the fact that when the heterotropic is seen in 
advance a lagging chromosome is not observed, and vice versa. It is, 
therefore, to be supposed that the heterotropic may pass indifferently 
either in front of the other chromosomes or behind them. 
SomE GENERAL CONSIDERATIONS.—As a result of our re-investigation 
of the spermatogenesis of Anax junius, it has been -established beyond 
doubt that this dragon-fly, in the behavior of the chromosomes of the 
male germ-cells, closely parallels the conditions which have been observed 
in some of the Hemiptera and other higher groups of insects. In the 
differentiation of its chromosomes as m-chromosomes, macro-chromo- 
somes, and chromosomes of intermediate sizes; in the occurrence of an 
odd number of chromosomes (27) in the male groups and of this number 
plus one (28) in the female groups; in the presence of an accessory or 
heterotropic chromosome which persists as a condensed body throughout 
the growth-period and passes undivided at the second maturation division 
into one of the spermatids, a strict parallelism may be recognized between 
Anax and those other insects, of which Anasa tristis may be taken as a 
type, which possess a heterotropic chromosome. In at least one of the 
Odonata, therefore, a dimorphism of the spermatozoa occurs, and the 
theory of the determination of sex by differentiated sex-chromosomes 
receives additional support from this group of insects. 
