756 DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
activity as digestive cells, and by an early development of this function in the embryo 
became capable of digesting the yolk-cells precisely as if the latter were foreign food- 
matters introduced through the mouth. How such a two-fold specialisation of the 
entoderm cells was possible is shown in the embryo of Gorgonia ; for in this case the 
yolk-cells still persist in an apparently functional state, being ciliated and surrounding 
a central cavity. Only a step before this is the planula of Gorgonia or Liriope 
(Mertscunikorr and For) in which the central cavity exists from the first and all the 
delaminated entoderm cells persist as such. 
If we push this speculation further and inquire after the causes which originally 
determined that some of the primitive entoderm cells should persist as such while 
others became yolk-cells, we encounter a very broad question, which it would be hardly 
profitable to enter upon here, since it belongs too exclusively at present to the region 
of pure speculation. The question is of the same nature, for instance, as that 
concerning the influence which determines the survival of a particular cell of the 
germinal epithelium of the ovary, as an ovum, while its neighbours are absorbed, or 
remain as simple epithelial cells.* We can only say that the differentiation probably 
stood in some relation with the relative position of the cells; for only the peripheral 
cells persist as entoderm cells. This suggests that the divergence may have depended 
upon, or is at least now directly determined by, differences in the supply of oxygen 
afforded to the cells—in other words is due to respiratory differences. The peripheral 
cells being nearer to the exterior, must command a more plentiful supply of oxygen, 
and in this respect have a decided advantage over the inner cells. This may be enough 
to determine the survival of the former and the disintegration of the latter. 
According to a theory of WrrssMANN’s, the cells of the ovary (in Leptodora) attain 
a certain ‘maximal development,” which is a critical point in the life of a cell. If it 
receive an additional impulse, though a very slight one, it continues to develop into 
an ovum at the expense of its less fortunate neighbours. If, on the other hand, it 
does not receive this impulse, the cell loses its power of development and is absorbed 
by the developing ova. The determining impulse is believed by WeErIssmANN to be 
a slight advantage of nutrition which is potent because acting at a critical moment. 
Such a theory of “maximal development” would seem to apply well in the present case, 
but the impulse to development does not seem to be in any way connected with general 
nutrition but only with the supply of oxygen. The theory, though resting perhaps on 
a rather slender basis, has the merit of showing how a very slight difference in the 
supply of oxygen might determine the survival or the degeneration of the cells. 
* See on this point Weissmann, ‘“ Ueber die Bildung von Wintereiern bei Leptodora hyalina,” Zeitz. f. 
Wiss. Zool., Bd. xxvii., 1876, who has given an elaborate discussion of the question in the case of the ova 
of the Cladocera. 
