4492 Insects. 



8. The heart is formed on the dorsal aspect between the mucous 

 and serous folds. In this way the details of development 

 closely correspond with those of the embryology of the other 

 Articulata which I have studied ; and the subject is all 

 the more interesting, as the germ-masses from which such de- 

 velopment occurs in no way and at no time structurally 

 resemble true eggs. 

 When the embryo is ready to burst from its developing capsule 

 and make its escape from the abdomen of its parent, it is about one- 

 sixteenth of an inch in length, or more than eight times the size of 

 the germ at the time when the first traces of development were seen. 

 From this it is evident that, even admitting that these germ-masses 

 are true eggs, the conditions of development are quite different from 

 those of the truly viviparous animals ; such as, for instance, in Musca, 

 Anthomyia, Sarcophaga, Tachina, Dexia, Miltogramma, and others 

 among Dipterous insects, or in the viviparous reptiles, — for in all 

 these cases of ordinary viviparity, the egg is simply hatched in the 

 body instead of out of it. The egg, moreover, is formed exactly in the 

 same way as though it was to be deposited, and its vitellus contains 

 all the nutritive material required for the development of the egg until 

 the coming forth of the new individual. The abdomen of the mother 

 serves only as a proper nidus or incubatory pouch for its full develop- 

 ment. This is true of all the ovo-viviparous animals whatsoever. 

 With the viviparous Aphides, on the contrary, the developing germ 

 derives its nutritive material from the fatty liquid in which it is 

 bathed, and which fills the abdomen of the parent. The conditions 

 of development here, therefore, are more like those in Mammalia, and 

 the whole animal may, in one sense, be regarded as an individualised 

 uterus filled with germs ; for the digestive canal, with its appendages, 

 seems to serve only as a kind of laboratory for the conversion of the 

 succulent fluids which the animal extracts from the tree on which it 

 lives, into this fatty 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 (poms genitalis) 

 situated just above the anus. In the species under consideration, 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 



