526 A. A. W. HUBREOHT. 



some important points. One of these has more particular 

 bearing on the question which we are here considering. It is — 

 does the hypoblast (constituted in these groups of much more 

 bulky cells than the epiblast) contribute towards the formation 

 of mesoblast in the ventral and caudal regions by actual de- 

 lamination from the surface ? 



Nuel, Scott and Osborn answer in the afiSrmative ; Shipley 

 and O. Hertwig in the negative. As the latter, however, very 

 distinctly admit and figure a participation of hypoblast-cells to- 

 wards the formation of mesoblast in the immediate vicinity 

 of the blastopore the question is more one of quantity than 

 of quality. Whether new cells are added from the hypoblast 

 to the mesoblast along circles of increasing radius, or whether 

 this increasing radius of the mesoblast is due to growth of the 

 free edge of the mesoblast, which was primarily derived from 

 hypoblastic cell matter, is in itself a phenomenon which may 

 be modified either one way or the other by more or less preco- 

 city of the segregation process.^ 



The importance of the phenomenon lies in the fact that a 

 special secondary portion of mesoblast (peristomales Meso- 

 blast, Rabl) originating from the hypoblast can also be traced 

 in the Amphibia, and that this portion must be homologous 

 to that which in Amphioxus is derived from the (similarly 

 hypoblastic) terminal " pole-cells of the mesoblast.^^ We have 

 seen above that the anterior part of the mesoblast in front of 

 the blastopore (gastrales mesoblast, Rabl), as it originates in 

 Amphibia, allows of very close comparison with the similarly 

 situated mesoblastic diverticula of Amphioxus, a comparison 

 which was firmly established by the most valuable investigations 

 of O. Hertwig. 



Turning to tiie Mammalia, we have seen that the com- 

 parison of the medio-dorsal mesoblast-wiugs (gastrales meso- 



1 Osc. Schultze regards tlie plieuomeiia as they present tlieniselves in 

 Amphibia in a very different liglit from Hertwig's. I will not here enter 

 upon the points of dispute between them, but merely remark that the 

 figures in both his papers would allow of an interpretation of the formation of 

 ventral and posterior mesoblast in the sense of Scott (for Cyclostomata), and 

 of Scott and Osborn (for Amphibia). 



