98 Dr. Burnett on the Development of Vivipa7'ous Aphides. 



proper, but simply the ordinary gemmiparous form already de- 

 scribed. Moreover, so great was the similarity of appearance 

 between these two forms — true females and gemmiparous indivi- 

 duals — that they could be distinguished only by an examination 

 of their internal genitalia. Among the proper females there were, 

 besides tliose which were filled with eggs or had already depo- 

 sited them, other individuals in which the ovaries were but feebly 

 developed, or at least, in which no mature eggs had been formed. 

 An opportunity was thereby afforded me to examine the struc- 

 tural differences between the true ovaries and their quasi repre- 

 sentatives — the bud-like processes. The true ovaries had their 

 usual, well-known structure — multilocular tubes containing nu- 

 cleated cells which are probably the undeveloped germs ; the 

 bud-like processes, on the other hand, consisted of a row of cell- 

 masses, oval and connected by a kind of peduncle, as described 

 in detail in the preceding paper. These wide differences have, 

 more than ever, persuaded me of the morphological dissimilarity 

 of these two kinds of reproducing parts in this animal. It seems 

 to me then that the real intrinsic difference between an ovum 

 and a bud lies as deep as the conditions of sex itself, notwith- 

 standing the latter often has, as in the present case, for instance, 

 some of the morphological characteristics of the former. 



The appearance of sexless, gemmiparous individuals in the 

 terminal brood would seem to indicate, moreover, that the con- 

 ditions which determine the appearance of individuals usually 

 exclusively male and female, are not, perhaps, referable to the 

 fact of this being the last brood, but rather to relations of warmth 

 and nutrition. This view is rendered more probable by the fact 

 of the variation in the number of broods between the first and 

 last, observed in the same species in different years — ranging 

 between seven, nine, eleven or more. Moreover, Kyber, as quoted 

 already in the preceding paper, by nursing continually in a warm 

 room a collection of Aphis dianthi, keeping about them a sum- 

 mer temperature, succeeded in continuing uninterruptedly the 

 series of sexless or gemmiparous individuals for four years. There 

 are many other facts in insect life that indicate in like manner 

 some direct relation between temperature and nutriment, and 

 definite sexual development. The subject is as important as it 

 is interesting in physiology, and these very animals will, perhaps, 

 subserve the successful studyof the primary morphological con- 

 ditions of sex. 



Iff) 



