Dr. Burnett on the Development of Viviparous Aphides. 83 



brood were, throughout, of the producing kind, no males having 

 been found upon the closest search; they were all, moreover, 

 winged ; and those few that were seen without these appendages 

 appear to have lost them by accident. I mention this fact espe- 

 cially, since it has been supposed by naturalists that the females 

 were always wingless, and therefore that the winged individuals, 

 or the males, appeared only in the autumn*. 



The first brood, upon their appearance from their winter hiding- 

 places, were of mature size, and I found in them the developing 

 germs of the second brood quite far advanced. On this account 

 it was the embryology of the third series or brood alone that I 

 was able to trace in these observations. 



A few days after the appearance of these insects, the indivi- 

 duals of second brood (B), still within their parents (A), had 

 reached two-thirds of their mature size. At this time the arches 

 of the segments of the embryo had begun to close on the back, 

 and the various external appendages of the insect to appear pro- 

 minently ; the alimentary canal had been more or less completely 

 formed, although distinct abdominal organs of any kind belonging 

 to the digestive system were not very prominent. At this period, 

 and while the individuals of generation B are not only in the 

 abdomen of their parent A, but are also enclosed, each, in its 

 primitive egg-like capsule, — at this time, I repeat, appear the first 

 traces of the germs of the third brood (C). 



These first traces consist of small egg-like bodies arranged two, 

 three, or four in a row, and attached in the abdomen at the locality 

 where the ovaries are situated in the oviparous forms of these ani- 

 mals. 



These egg-like bodies consisted either of single nucleated cells, 

 of ^ij^oTJ^^ ^^ ^^^ ^^^^ ^^ diameter, or a small number of such cells 

 enclosed in a simple sac. These are the germs of the third gene- 

 ration; they increase with the development of the embryo in which 

 they have been formed, and this increase of size takes place, 

 not by a segmentation of the primitive cells, but by the endo- 

 genous formation of new cells. After this increase has gone on 

 for a certain time, these egg-like bodies appear like little oval bags 

 of cells — all these component cells being of the same size and 

 shape, there being no cell which is larger and more prominent 

 than the rest, and which could be comparable to a germinative 

 vesicle. While these germs are thus constituted, the formation 

 of new ones is continually taking place. This occurs by a kind 



* See Westwood, An Introduction to the modern Classification of In- 

 sects, &c. London, 1839, ii. p. 438 ; but especially Owen, Partheno- 

 gene.>is, &c., p. 23, note, and p. 59, note, where he says, " Many of the 

 virgin viviparous Aphides acquire wings, but never perfect the generative 

 organs ! " 



