and Embryogeny of the Aphides. 67 
culata, containing only the elements of the head and of the in- 
ferior wall of the body. The sexual masses are therefore, in 
point of fact, completely naked and outside the embryo. But 
already we can see that the embryonic cells arrange themselves 
in parallel series directed towards the extremity of the abdo- 
men, to form excretory ducts, destined to place them in relation 
with that region. 
Development continuing under these conditions, the embryo 
enlarges, and with it the whole sexual apparatus; the parts al- 
ready existing become completed and perfected, the mouth and 
anus are formed, and the digestive tube becomes visible at its 
extremities. It is at this pomt that a phenomenon occurs 
equally simple in its mechanism and important in its results 
for the further progress of the development of the Articulata, 
namely the reversal of the mode of rolling of the embryo. This 
reversal, which is not always effected by means of exactly the 
same process in all these animals, takes place in the Aphides by 
an actual backward summerset executed by the embryo in the 
interior of its cell. In consequence of this change of position 
the head, which at first was in relation to the posterior part of 
the cell, arrives at the anterior part, while the ventral surface, at 
first turned inwards, now looks outwards, and is placed imme- 
diately under the envelope of the egg. By the same act the 
abdomen is transferred to the dorsal side, and rises, like a sort 
of tail to the embryo, as far as the posterior part of the head, 
leaving between it and the cephalothoracic rudiment a space, 
the greater part of which is occupied by the mass of the gene- 
rative organs. In this new position all that is necessary is that 
the embryo should complete itself behind by the formation of a 
dorsal wall, in order that these organs may be enclosed in the 
cavity of the body quite naturally and without any new change 
of position. 
The closure of the hinder part of the bedy is effected by the 
simple growth of the ventral arches towards the dorsal region, 
and their fusion in the median line of the latter. 
If we endeavour at this period of development to under- 
stand the arrangement of the hermaphrodite apparatus of the 
viviparous Aphides, we find the primitive common mass of ova- 
rian cells divided into two groups, symmetrically placed in the 
posterior part of the body, and each of these groups formed of 
a small number of cellular masses, each of which possesses a 
proper envelope. In these we easily recognize the terminal 
chambers of the ovarian sheaths, with their contents, consisting 
of small transparent cells. The male organ is likewise divided 
into two parts, arranged, in the form of two cords of variable 
form, on each side of the digestive tube, within the ovaries, 
