446 WALTER HEAPE. • 



as an inpushing of the epiblast, and connects the neural tube 

 with the gastric cavity. 



Kolliker (No. 17) agrees with Lieberkiihn as to the meso- 

 blastic origin of the notochord. 



Summary. — My own work indicates that a complete neuren- 

 teric canal is formed similar to that in birds and lizards, first 

 of all by an inpushing of the epiblast; secondly, the canal is 

 conducted to the hypoblast within a tongue of mesoblast, which 

 grows from the anterior end of the primitive streak ; thirdly, the 

 canal enters the axial hypoblast, and opens below to the cavity 

 of the vesicle; and fourthly, the dorsal opening of the neu- 

 renteric canal is eventually enclosed within the walls of the 

 neural tube. 



With regard to the notochord, it appears to me evident that it 

 is an hypoblastic structure, since it arises from an axial mass 

 of cells, which are themselves derived from the primitive hypo- 

 blast. 



My observations are at variance with Schafer's, in that 

 I find no continuous layer of mesoblast in front of the medullary 

 groove, such as he describes, but a mass of undifferentiated 

 cells, whose development shows that they are of hypoblastic 

 origin, and that they split up laterally into sheets of hypoblast 

 and mesoblast, while axially they remain undifferentiated, and 

 give rise to the notochord. 



Further, that the differentiation of the mass of cells which 

 gives rise to the notochord takes place, as does the first forma- 

 tion of the medullary groove, from behind forwards, but that 

 the separation of the notochord from the hypoblast takes place 

 first of all somewhat anterior to the middle of the embryo, in 

 the region where the medullary groove first deepens, and where 

 the lateral mesoblast first forms protovertebra?, and that from 

 that point the notochord is separated ofi" both backwards and 

 forwards. 



Finally, that the derivation of the notochord from hypoblast 

 is still further evidence of tlie incompatibility of Repiachoff's 

 views (loc. cit.) with the facts of development. 



