^^2 JORDAN. [Vol. VIII. 



in support of the splitting hypothesis. This evidence is, un- 

 fortunately, almost wholly of a negative character, and concerns 

 itself with showing that the appearances observed in sections 

 do not necessarily indicate invagination, but tally equally well 

 — or, as is believed, better — with the hypothesis of delamina- 

 tion. The direct evidence for delamination seems to me 

 singularly inadequate. I confess that it is difficult for me to 

 understand why so much stress should be laid by the advocates 

 of delamination upon the fact of the equality of cells, yolk 

 granules, and pigment at the first origin of the archenteric 

 cleft. While this fact does perhaps leave open the possibility 

 of a subsequent differentiation of these elements in situ, it in 

 no way indicates the probability of such an occurrence. It 

 might be urged by the upholders of invagination, with perfect 

 justice, that, although the first step in the formation of the 

 archenteric cleft may be a split between the yolk-cells, there is 

 no reason why one should not suppose invagination immediately 

 to follow. There is not a shred of evidence to show that the 

 large cells at first surrounding the mouth of the blastopore are 

 not subsequently pushed in by the ingrowth of ectoblast cells. 

 No positive evidence whatever exists to prove either the impos- 

 sibility of invagination or the likelihood of no-invagination. I 

 find it difficult to gather the reasons that have influenced 

 Houssay, and Robinson and Assheton to adopt the view that 

 invagination does not occur. Even if some of the usual argu- 

 ments for invagination have no weight and have been carelessly 

 advanced, that fact would hardly prove that the process of 

 invagination did not occur. 



I must here advert to the elementary fact that there are 

 really two processes of "invagination" concerning which it is 

 important to distinguish clearly. " Invagination " may mean 

 either that the upper small (ectoblast) cells grow down over the 

 large cells of the yolk-plug, or that the cells around the rim of 

 the blastopore are turned in and rolled under to form the wall 

 of the archenteron. Now either one of these invaginative pro- 

 cesses may occur alone, or both may occur together. If I 

 understand Robinson and Assheton correctly they maintain 

 that the small cells neither grow down nor fold in {Rana). 



