OF ARTS AND SCIENCES. 57 



those of the development of true oviparous Arthropoda in general ; 

 and although the ovoid germ has at no time the structural peculiari- 

 ties of a true ovum, — such as a real vitellus, germinative vesicle, and 

 dot, — yet if we allow a little latitude in our comparison, and regard 

 the vitellus-looking mass as the mucous, and the germ-mass proper as 

 the serous fold of the germinating tissue, as in true ova ; — if this com- 

 parison of parts can be admitted, then the analogy of the secondary 

 phases of development between these forms and true ova of the 

 Arthropoda can be traced to a considerable extent. 



" These secondary phases of development need not here be detailed, 

 for they correspond to those described by Herold and KoUiker, of 

 the true ovum in other Insecta, and which, too, I have often traced 

 in various species of the Arthropoda in general. 



" When the embryo is fully formed and ready to burst from its 

 capsule in which it has been developed, it is about one sixteenth of an 

 inch in length, or more than eight times the size of the germ, when 

 the first traces of development in it were seen. From this last-men- 

 tioned fact, it is evident that, even admitting that these germ-masses 

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

 those of the eggs of the truly viviparous animals, for in these last 

 the egg is merely hatched in the body instead of out of it, and, more- 

 over, it is formed exactly as though it was to be deposited, and its 

 vitellus contains all the nutritive material required for the develop- 

 ment of the embryo until hatched. With the Aphididse, on the other 

 hand, 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 in this respect are here, therefore, 

 more like those of the Mammalia, and the whole parent animal may 

 be regarded in one sense as an individualized 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 liquids this 

 animal extracts from the tree on which it lives, into this fatty liquid 

 which is the nutritive material of the germs. 



" Omitting the curious and interesting details of the further history 

 of the economy of these Insecta, as irrelevant to the point in discus- 

 sion, we will now turn to see what view we should take of these pro- 

 cesses, and what is their physiological interpretation. In the first 

 place, it is evident that the germs which develop these viviparous 

 Aphides are not true eggs ; they have none of the structural charac- 



VOL. III. 8 



