ON THE PROCESS OF GASTRULATION IN CHELONIA. 275 



On another point I would like to say a few words. Referring to 

 the structure in Mammalia which \^an Beneden homologizes with the 

 yolk-plug in Amphibia, Keibel says : — ^' Der Dotterpfropf hat bei den 

 Ami)hibien keine grosse morj)hologische Bedeutung, er ist ein Ent- 

 wicklungs h i n d e r n i s s, er hat kein Funkti • )n. Warum sollte gerade 

 diese Bildung so zäh festgehalten werden, während doch so vieles 

 Andere, das von ungleich grösserer Bedeutung ist, undeutlich wird 

 und verschwindet." As I have insisted on the presence of the yolk- 

 plug in Iveptilia, ever since Ishikawa and I tirst discovered it. I may 

 perhtips give my own view on Keiljel's objection. I have in a 

 previous ]jage tried to explain the chang.^ of shape in the blastopore 

 in Reptilia as repeating that process by wliich the lower half of the 

 Amphibian egg becomes enclosed by tlie epiblast. Taking this in 

 connection with the presence of the yolk-plug, I think that wh.at is 

 inherited is not simply the yolk-plug but the whole process of the 

 epiboiic invagination which is gone through in the region of the 

 primitive plate. This is certainly important enough to persist for a 

 long time. 



Tokyo, Oct. 1893. 



