464 THE INSECTA. § 355. 
wuderma, which corresponds to the ventral side of the future embryo, extends 
gradually in all directions and at last encompasses the whole vitellus, — its 
borders meeting on the dorsal surface. It may be divided into an exter- 
nal or serous, and an internal or mucous 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 Lachnus 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 appearance of the first colony 
(A), the second colony (B), still within the former, 
had reached two-thirds 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 
distinet 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 time, 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 
thr2e-thousandth of an inch in diameter, or a small 
number of such cells enclosed in a simple sac. 
These are the germs of the third generation or 
colony, and they increase pari passu 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 sarc, 
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 [ 
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 multiplied 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 gern) proper. 
This yellow mass increases after this period, paré. 
possu with the germ, and at last lies like a cloud 
over and partially concealing one of its poles. L 
would, moreover, insist upon the point that it does. 
not gradually extend itself over the whole germ 
. 
