84 Dr. Burnett on the Development of Viviparous Aphides. 



of constriction-process of the first germs, one of tlieir ends being 

 pinched oif, as it were, and in this way what was a single sac, 

 is changed into two which are attached in a moniUform manner. 

 The new germ thus formed may consist of even a single cell 

 only, as I have often seen, but it (the germ) soon attains a more 

 uniform size by the endogenous formation of new cells within 

 the sac by which it is enclosed. In this way the germs are mul- 

 tiplied to a considerable immber, the nutritive material for their 

 growth being apparently a fatty liquid with which they are bathed, 

 contained in the abdomen, and which is thence derived from the 

 abdomen of the first parent. 



When these germs have reached the size of ^^^(jth of an inch in 

 diameter, there appears on each, near one end, a yellowish vitel- 

 l us -looking mass or spot, which is composed of large yellowish 

 cells, which in size and general aspect are different from those 

 constituting the germ proper. This yellow mass increases pari 

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

 cealing one of its poles. I would also insist on the point that it 

 does not extend itself gradually over the whole germ- mass, and 

 is therefore quite unlike a true germinative vesicle or a prolige- 

 rous disc. When the egg-like germs have attained the size of 

 y-^jjth of an inch, there distinctly appears the sketching or mark- 

 ing out of the future animal. This sketching consists at first of 

 delicately-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 only of rows of simple cells. This point is 

 here beautifully prominent, and nowhere have 1 observed finer 

 illustrations of the cell-constitution of developing forms. 



The development thus proceeding, each part of the dermo- 

 skeleton becomes more and more distinct, and the increase of size 

 of the whole is attained by the constant development of new cells. 

 During this time, the yellow vitellus-looking mass, situated at one 

 of 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 further, and the embryo 

 is pretty well formed, the arches of the segments, which have 

 hitherto remained gapingly open, appear to close together on the 

 back, thereby enclosing this vitellus-looking mass within the ab- 

 dominal cavity. 



It is this same vitelloid mass thus enclosed that furnishes the 

 nutritive material for the development of new germs which would 

 be those of the fourth brood, or D ; this development of germs 



