464 



THE IXSECTA. 



*^ 355. 



toderma, which corresponds to the ventral side of the future embryo, extends 

 gradually in all directions and at last encomjjasses the whole vitellus, — its 

 borders meeting on the dorsal surface. It may be divided into an exter- 

 nal or serous, and an internal or nmcous layer. In the first of these is de- 

 veloped, on the median abdominal line, the ventral cord ; while the second 

 forms a semi-canal which gradually surrounds the vitellus and at last com- 

 pletely enveloping it, is changed into the digestive canal. The various ap- 

 pendages of this canal are subsequently formed by simple constrictions or 

 deverticula from its cavity ; while the other abdominal viscera are directly 

 developed from a special blastoderma. 



Upon the external surface of the serous layer are formed the parts of the 

 mouth, the' tactile organs, the legs, and the other appendages of the body, 

 whose articulations, like those of the body itself, are produced by constric- 

 tions. 



The dorsal vessel is formed between the two blastodermic layers on the 

 side opposite that of the ventral cord. This development of the embryo 

 takes place at the expense of the vitellus, which, enclosed in the digestive 

 canal, is gradually consumed.^ 



* [ End of § 355.] The subject, which has been 

 frequently alluded to in this book, — -the singular 

 mode of reproduction of the viviparous Aphididae, 

 is one of so much interest and importance in physi- 

 ology, that I propose to give it something more 

 than a brief mention. Moreover, I have enjoyed 

 excellent opportunities for the study of these phe- 

 nomena in question, and have advanced an inter- 

 pretation of them, and their like elsewhere, quite 

 different from that usually received. 



My observations were made upon Aphis caryae 

 (probably Laclinus of Illiger, or Cinara of Cur- 

 tis), one of the largest and most favorable species 

 for these investigations. This was in the spring of 

 1853. The first colony, on their appearance from 

 their winter quarters were of mature size, and con- 

 tained, in their interior, the developing forms of the 

 second colony quite far advanced in formation. On 

 this account it was the embryology of the third 

 series or colony, that I was able to first trace. A 

 few days after the api)earauce of the first colony 

 (A), the second colony (B), still within the foi'mer, 

 had reached two-thii-ds of their full embryonic size ; 

 the arches of the segments had begun to close on the 

 dorsal surface, and the various appendages of the 

 embryo were becoming prominent ; the alimentary 

 canal was more or less completely formed, although 

 distinct abdominal organs of any kind belonging to 

 the digestive system were not apparent. 



At this time, and while the individuals B. were 

 not only in the abdomen of their parents A., but 

 were also enclosed each in its primitive egg-like 

 capsule ; at this tune, I repeat, appear the first 

 traces of the germs of the third colony, C. Their 

 first traces consisted of small egg-like bodies, ar- 

 ranged two, three, or four in a row, and attached 

 at the locality where are situated the ovaries in the 

 oviparous forms of the Aphididae. These egg-like 

 bodies were either single nucleated cells of one 

 three-thousandth of an inch in diameter, or a small 



number of such cells enclosed in a simple sac. 

 These are the germs of the tliird generation or 

 colony, and they increase pari pasmi with the de- 

 velopment of the embryo in which they are formed, 

 and this increase of size takes place not by the 

 segmentation of the primitive cells, but by the en- 

 dogenous formation of new cells within the sac. 

 After this increase has continued for a certain time, 

 these bodies appear like little oval bags of cells, — 

 all the component cells being of the same size and 

 shape, — there being no one particular cell which 

 is larger and more prominent than the others, and 

 which could be comparable to a germinative vesi- 

 cle. While these germs are thus constituted the 

 formation of new ones is continually taking place. 

 This occurs by a kind of constriction-process of 

 the first germs ; one of the ends of these last being 

 pinched off, as it were, and so, what was before a 

 single body or sac, becomes two which are attached 

 in a moniliform manner. The new germs thus 

 formed may consist each of a single cell only, as I 

 have often seen ; but they soon attain a more uni- 

 form size by the endogenous formation of new cells 

 within the sac in which it is enclosed. In this way 

 the germs are multiphed to a considerable number, 

 the nutritive material for their growth being, ap- 

 parently, a fatty liquid in which they are bathed, 

 contained in the abdomen, and which is thence de- 

 rived from the abdomen of the first parent. When 

 these germs have reached the size of about one 

 three-hundredth of an inch in diameter, there ap- 

 pears on each, near the inner pole, a yellowish, 

 vitellus-looking mass or spot, composed of yellow- 

 ish cells, which, in size and general aspect, are dif- 

 ferent from those constituting the germ proper. 

 This yellow mass increases after this period, pari 

 passu with the germ, and at last lies like a cloud 

 over and partially concealing one of its poles. I 

 would, moreover, insist upon the point that it does 

 not gradually extend itself over the whole germ- 



