ON THE PROCESS OP GASTRULATION IN CHELONIA. 261 



der Säiigethiere der Gastrulution d. h. der Eiastiilpung, die man von 

 der Epibolie auseinanderhalten muss, vorangeht, und das s die ZAvei 

 Schichten respektiv dem Ektoderm und dem Entoderm 

 des Amphioxus nicht entsprechen. Dieser Schhiss geht 

 schön daraus hervor, dass nicht allein die Organe des Epiblastes, 

 sondern auch die Chorda und der ganze Mesohlast aus der äussern 

 Schicht sich bilden." According to my views, the epiljlast of the 

 Am ni Ota is liomologous with the epiljlast of Amphioxus. The 

 difficulty which keeps Van P>eneden from accepting this idea lies in this, 

 that not having for comparison the comparatively simple story of the 

 reptilian development he has reckoned as epiblast what corresponds to 

 the primitive plate of Reptilia. If he had recognised the structure 

 which, as 1 have sliown above, can not be said to belong to either layer 

 and then considered the lower layer as precociously developed hypoblast, 

 the conclusion would h:ive been inevitable that the outer layer corres- 

 ponds to the epil)last of Amphioxus . Keibel (Xo. 7) has also shown 

 to what contradiction Van Beneden's theory of the " Blastophor " and 

 " Lecitophor " leads. I think, however, I have now removed the 

 second objection of Keibel:—" Dazu kommt dann ferner, dass uns 

 Van r>eneden den Beweis dafür durchaus schuldig gebhei^en, dass nun 

 wirklich die untere Schicht des zweischichtigen Säugethierkeimes und 

 die Keimhöhle desselben mit der Bildung des definitiven Darms der 

 Säuo-er nichts zu thun hat." I think, the fact that the whole yolk- 

 sac with the subgerminal cavity within it does not form in Chelonia 

 any permanent part of the alimentary canal, makes it highly probable 

 that the same is also true of the homologous structure in Mammalia. 

 As I have more than once stated above, I accept Hubrecht's view of 

 precocious segregation. In many respects my views are very much 

 like his, but I do not think, he makes a clear distinction between the 

 Archenteron and the yolk-sac. Nor do I know from personal 



