Dr. Burnett on the Development of Viviparous Aphides. 65 
does not extend itself gradually over the whole germ-mass, and 
is therefore quite unlike a true germinative vesicle ora rolige- 
rous disc. When the egg-like germs have attained the size of 
r#5 Of an inch, there distinctly appears the sketching or marking 
out of the future animal. This sketching consists at first of del- 
icately-marked retreatings of the cells here and there, but which 
soon become more prominent from furrows, and at last the 
whole form of the embryo stands boldly out. As the whole idea 
and form of the insect is thus moulded out of a mass of cells, 
it is evident that the separate parts which then appear, such as 
the arches of the segments, the extremities and the oval appa- 
ratus, consist at first of only rows of simple cells. This point is 
here beautifully prominent, and nowhere have I observed finer 
illustrations of the cell-constitution of developing forms. 
he development thus proceeding, each part of the dermo-skel- 
eton becomes more and more distinct, and the increase of size o 
the whole is attained by the constant development of new cells. 
During this time, the yellow vitellus-looking mass, situated at one 
ot the poles of the embryo, has not changed its place ; it has in- 
creased somewhat in size, but otherwise appears the same. When 
the development has proceeded somewhat farther, and the embryo 
s pretty well formed, the arches of the segments, which have 
itherto remained gapingly open, appear to close together on the 
ack, thereby enclosing this vitellus-looking mass within the ab- 
minal cavity, 
tis this same vitelloid mass thus enclosed that furnishes the 
hutritive material for the development of new germs which would 
those of the fourth brood or D; this development of germs 
here Commences with the closing up of the abdominal cavity, 
and the same processes which we have just described are again 
repeated, 
_ The details of the development subsequent to this point, are 
like those of the development of ordinary insects or of the Ar- 
eulata in general ; and although this ovoid germ has at no time 
the structural peculiarities of a true ovum—such as a real vitellus, 
4 germinative vesicle and germinative dot; yet, if we allow a 
little latitude in our comparison and regard the vitellus-looking 
Mit this comparison of parts, then the analogy of development 
tween these germs and true eggs of insects, may be traced in 
Sonsiderable detail. j 
f 'S Comparison I have been inclined to admit at least in part, 
fom the Striking resemblance of these developing forms at cer- 
he stages, with the embryological forms of spiders as they have 
D figured by Herold* and as I have myself traced them. 
* Herold, De Generatione Aranearum in ovo. Marbourg, 1824. 
Szates, Vol. XVII, No. 49.—Jan,, 1854. ee 
