Dr. Burnett on the Development of Viviparous Aphides. 87 



liquid from which the increase and development of the germs 

 take place. 



When the young animal has reached its full development as 

 an embryo, it bursts from its encasement and appears to escape 

 from the abdomen of its parent through a small opening (porus 

 genitalis) situated just above the anus. In the species under con- 

 sideration it generally remains clinging on the back of the parent 

 until its external parts are dry and it is able to begin life for itself. 

 Each parent here produces from eight to twelve individuals, and 

 if this rapid increase is continued undisturbed, through seven to 

 nine broods, we cannot wonder at the countless numbers which 

 appear from so few original individuals *. 



Such are the details of the embryological development of the 

 so-called viviparous Aphides, as far as I have enjoyed opportu- 

 nities for their study. We will now refer for a moment to the 

 special points which have here been made out. In the first place, 

 it is evident that the germs which develope these forms are not true 

 eggs. They have none of the structural characteristics of eggs, 

 such as a vitellus, a germinative vesicle and dot ; on the other 

 hand, they are, at first, simple collections, in oval masses, of nu- 

 cleated cells. Then again, they receive no special fecundating 

 power from the male, as is the necessary preliminary condition of 

 all true eggs ; and, furthermore, the appearance of the new indi- 

 vidual is not preceded by the phenomena of segmentation, as 

 also is the case with all true eggs. Therefore their primitive 

 formation, their development, and the preparatory changes they 

 undergo for the evolution of the new individual, are all different 

 from those of real ovaf. 



Another point is, these viviparous individuals have no proper 

 ovaries and oviducts. Distinct organs of this kind I have never 

 been able to make out. The germs are situated in moniliform 

 rows, like the successive joints of confervoid plants, and are not 

 enclosed in a special tube. These rows of germs commence, 

 each, by a single germ-mass which sprouts from the inner surface 

 of the animal, and which increases in length and in the number 



* Reaumur has shown that in these animals the rate of increase is so 

 great, that in five generations or broods only one Aphis may be the proge- 

 nitor of five billion nine hundred and four million nine hundred thousand 

 (5,904,900,000) descendants ; we may well ask, what would be the number 

 of descendants where the broods were extended to eleven ! ! — See Kirby 

 and Spence, Introduction to Entomology, i. p. 175. 



f Milne-Edwards thinks he has found true ova and ovaries in the vivi- 

 parous forms of these animals. (Quoted by Dr. Carpenter in Brit, and For. 

 Med. Chir. Rev. 1849, iv. p. 443.) I think he must have been deceived. 

 as I was at first, by the general appearances, which, unless careful 1\ ex- 

 amined, closely resemble those of true oviparous individuals. 



