ox Tue process OE UASTRULATIOX IX CHELOXIA. 257 



hypoblast. That it arises before the invagination of the archen- 

 teron can be explained by the principle of precocious segrega- 

 tion, as has been pointed out by Hubrecht (No. 5). The name 

 "cœnogenetic hypoblast" which he applies to this layer, seems 

 therefore very appropriate as the part derived Ijy invagination may be 

 called the "palingenetic hypoblast." 



I think the objection on the part of Robinson and Asshetoii 

 (No. 10) that the yolk-})lug can not be present at the spot designated 

 by Ishikawa.and myself is fully answered by comparing the two 

 diagrams (Woodcuts I. & II.). The yolk-plug can not only pro- 

 perly be found at this spot but its presence here is one of the signi- 

 ficant, although not the essential, features in homologizing it with the 

 amphibian egg. This is an example of those cases where a secondary 

 characteristic is of great service in identification. 



According to the views set forth above, the enormous accumula- 

 tion of yolk has profoundly affected the course of development in the 

 chelonian agg, especially in the precocious develo})ment of a part of 

 the hypoblast, and in the rapid spreading of the blastoderm over the 

 surface of the egg. There is left, however, in the centre of 

 the blastoderm a certain amount of raw undiiferentiated 

 materials in the shape of the primitive ])late or knob 

 in order to go with it through certain developmental processes of 

 palingenetic character : — the invagination of the archenteron with the 

 consequent establishment of the cliorda-hypoblast, the peristomal and 

 gastral mesoblast, the yolk-plug, and the definitive hypoblast. The 

 changes in the shape of the blastopore from a crescent with its con- 

 cavity turned anteriorly to that of a horse-shoe with its two limbs 

 directed backwards and enclosing the yolk-plug between them nnist 

 be looked on as the remnant of that process by which the epiblast 

 gradually encloses the endoderm cells in the amphibian ovum or the 



