No. 2.] THE DEVELOPMENT OF THE NEWT. 33I 



of the yolk-plug. This distinction, however, does not exist in 

 early stages of the archenteron, as has been well shown by 

 Houssay for the axolotl. I have observed a similar condition 

 in Dicmyctylus : at the beginning of the formation of the 

 archenteron the yolk granules in the surrounding cells are prac- 

 tically identical in size, and it is only in later stages that the 

 yolk granules are smaller in the dorsal cells.^ This reduction 

 in the size of the yolk granules may perhaps depend, like the 

 size of the cells and the presence of pigment, upon the 

 activity of the cells themselves and may have been due to the 

 differentiation of cells in situ. The size of the particles of 

 yolk is consequently not a sound criterion of the source of the 



cells. 



No one, of these characteristics of the cells of the dorsal 

 wall, therefore, can be regarded as in any way satisfactory 

 evidence for infolding : neither the size of the cells nor that of 

 the yolk granules, and assuredly not the accumulation of pig- 

 ment. All of these appearances may be explained perhaps 

 equally well by supposing a differentiation of the cells itt situ. 

 Houssay regards the difference between the dorsal and ventral 

 walls as due simply to a difference in rapidity of development, 

 and believes that the ventral cells pass through the same stages 

 as the dorsal, but more slowly. Why there should be this 

 striking disparity he does not attempt to explain. 



The evidence thus far adduced for invagination is, to say the 

 least, inconclusive. Let us next examine that brought forward 



1 Houssay, taking his cue from Nuel ('81), believes in the subsequent regener- 

 ation of these reduced yolk granules ! " II y a plus. Ces cellules de la parol 

 dorsale, qui avaient d'abord une apparence identiques aux cellules vitellines, puis 

 qui ont acquis une forme voisine de celle des elements epiblastiques, vont de 

 nouveau perdre cet aspect, acquerir de nouveau une grande taille et de gros 

 granules, en passant de I'etat d'activite qui preside a la formation de I'intestin, a 

 I'etat de repos ou elles demeurent tant que dure la formation du systeme 

 nerveux" (p. 170). Houssay seems to ignore the grave physiological difficulties 

 that lie in the way of accepting such an independent existence for yolk granules ; 

 and it is certainly difficult to appreciate the precise advantage to the embryo of 

 removing nutritive material from one portion of the developing egg to deposit 

 it in the form of large yolk granules in the cells of the dorsal wall ! It is hardly 

 necessary to suggest the fallacy involved in comparing sections of different stages, 

 especially in such a respect as the size of yolk granules, and assuming continuity of 

 development. 



