OF PLANOCERA ELLIPTICA. 31 5 



centre towards the periphery, to expend the matter, to work it for its own purpose, 

 and model it accordino^ to its own wants and its own design. 



Vlir. 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 graduallv 

 increasing as shown by the growing opacity of the yolk (fig. I — 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. 

 Ii5, 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, 4S, 51, and 52). 



These floating cells are of two kinds, each kind originating in a diff'erent 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 hernia. 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- 

 what in size, and developed into epithelian cells, according to a general physiological 

 law. If, perchance, they congregate immediately on being detached from the yolk 



