16 
EMBRYONIC DEVELOPMENT 
VIII. 
THE VITELLINE SUBSTANCE. 
Meanwhile the egg is passing through its diverse periods, modifying the general 
aspect of its mass as a whole, the vitelline substance itself is undergoing a cellular 
process of multiplication and renewal. 
It has been remarked above that at the end of the period of the division of 
the yolk, and which precedes immediately the period of the embryo, the vitelline 
sphere had grown larger. In the ovarian egg the vitelline substance is gradually 
increasing as shown by the growing opacity of the yolk (fig. 1—6). 
When the egg has just been laid, the vitellus is composed of small homogeneous 
cells, though varying in size, but all provided with large nuclei (fig. 12, a). When 
divided into four spheres, the constituting cells have become heterogeneous ; there are 
large cells, each containing several nuclei, and small cells with but one nucleus (fig. 
35, a). Towards the end of the process of the division, the yolk cells are again small 
and more uniform amongst themselves (fig. 45, a). 
These facts plainly show that an intimate elaboration is taking place within each 
cell ; nuclei develope in them, which, by their own expansion, become perfect cells 
themselves, and in their turn producing internal nuclei. Cells, thus, increase in 
number, and by repeating the same process, accumulate in a larger and a more dense 
mass. 
IX. 
FLOATING CELLS. 
As soon as the egg has passed through the phases of the division, transparent cells 
make their appearance, floating in the albuminous zone, between the vitellus and the 
outer membrane of the egg (fig. 45, 46, 43, 51, and 52). 
These floating cells are of two kinds, each kind originating in a different manner. 
§ 1. The larger of these transparent cells are originally formed within the yolk, 
finding their way out as shown in fig. 47, where three of them appear near the sur- 
face under the form of transparent hernise. They once were vitelline cells, which 
outgrew their circle of activity under the influence of the central activity of the yolk. 
Their vital power being extinguished, they are now rejected as no longer useful. 
§, 2. The small ones, much more numerous than the former (fig. 52), were like- 
wise vitelline cells, which on being accidentally dropped from the surface of the yolk, 
and remaining in an isolated state without any work to perform, no longer under the 
influence of vitelline life, lost entirely their activity and vital power. They grew some- 
