No. 2.] THE DEVELOPMENT OF THE NEWT. 329 



teron is formed in situ by splitting amongst the yolk-cells, and 

 that it is entirely surrounded by modified yolk cells." (R. and 

 A.) The entoblast on the latter interpretation takes its origin 

 wholly from the so-called yolk-cells ; on the invagination hy- 

 pothesis it comes in part from infolding of the cells once 

 covering the exterior and is continuous with them at the lips 

 of the blastopore. 



It is evident at the outset that the question of invagination 

 or no-invagination can not readily be determined from the 

 inspection of sections. The most that can be derived from 

 sections will be only circumstantial evidence of greater or less 

 significance. In the opinion of some investigators there is to 

 be found in sections certain positive evidence that favors the 

 invagination hypothesis ; their opponents deny the validity of 

 such evidence and regard the appearances observed as ac- 

 counted for more easily on the splitting hypothesis. 



If we examine some of the evidence that has been supposed 

 to indicate invagination it cannot be said to carry conviction. 

 Some stress has been laid upon the fact that the cells lining 

 the dorsal wall of the archenteron, and lying opposite to the 

 cells of the yolk-plug, are smaller than those of the ventral 

 wall, and that by this character they show their affinity to the 

 ectoblast cells, with which, moreover, they are in direct con- 

 nection. It has been shown by Robinson and Assheton, how- 

 ever, that in Rana this difference in cell size does not appear 

 in the early stages of the blastopore, but becomes manifest 

 only after the blastopore is well established. At first there is 

 practical equality in the size of the cells around the blasto- 

 pore and only later can a difference in size be detected. 

 This is also the case in Dietnyctyhis. Later on, however, there 

 is a striking inequality of size (Fig. 52). The advocates of 

 the splitting hypothesis (v. Houssay, '90, p. 165) attribute 

 this inequality to the greater activity of the cells forming 

 the dorsal wall of the archenteron. That is to say, the dor- 

 sal archenteric cells divide more rapidly than the cells below 

 them, and hence their number is greater and their average size 

 less ; this differentiation is supposed to have taken place iii 

 situ. It is obvious that this is a plausible or at any rate 



