108 On the Reproduction and Embryogeny of the Aphides. 
developed testis; but in the female it is otherwise, and we shall 
see, in speaking of the development of the egg, that its presence 
in this sex has a much more important signification. 
The conditions which influence the determination of the sexes 
in the Aphides are probably of the same kind as those which 
act in a more general manner to bring on a change in their 
mode of propagation ; that is to say, they are probably depen- 
dent upon the phenomena of nutrition in these insects. The 
following observations support this opinion. 
At the period when the production of the dicecious genera- 
tions commences we find that at first females are almost exclu- 
sively generated, the males being still comparatively rare. But 
the latter soon become more and more numerous, and at last 
are even produced in greater abundance than the female indivi- 
duals. A single hermaphrodite mother may, moreover, contain 
at the same time embryos of both sexes, succeeding each other 
without apparent order in the interior of her ovarian sheaths. 
It is curious to observe the difference of coloration of the male 
and female embryos of the same species. The latter alone pre- 
sent a colour which resembles that of their mother: thus, for 
example, in a species of which the viviparous individuals are 
brown, the oviparous females are also brown, whilst the males 
are constantly green*, and vice versd. This difference of colour 
is due to the oleaginous globules which fill the cells of the fatty 
body, and is, no doubt, connected with a different chemical 
composition of the nutritive fluids in the embryos of the two 
Sexes. 
After this brief exposition of the embryogenic phenomena 
connected with the determination of the sexes in the Aphides, 
it remains for me, in order to traverse the whole reproductive 
cycle of these animals, to describe in few words what I have 
been able to observe of the development of the ovum destined to 
reproduce the viviparous generations with which we commenced 
this vestigation. Notwithstanding the considerable differences 
as to its elementary constitution and the conditions of its deve- 
lopment presented by the voluminous ovum of the oviparous 
Aphides when compared with the little ovule of the viviparous 
individuals, there is nevertheless a striking analogy in the phe- 
nomena of which both are the seat. Although the formation of 
the embryo does not commence in the former until after it has 
been fecundated by the male and brought into the world, it 
nevertheless, whilst still enclosed in the ovary, exhibits pheno- 
mena which indicate that genetic operations have already begun 
in its interior. Thus we observe, at the posterior pole of this 
* At least in the embryonic and larval states; the adult male is aimost 
always blackish. 
