524 A. A. W. HUBRECHT. 



78), and I have no doubt that this small strip of tissue has 

 actually been eliminated, when once the protochordal wedge has 

 become fused with the protochordal plate, and when the meso- 

 blast extends as a continuous sheet (though of multiple origin) 

 between epiblast and hypoblast. The gastrula ridge and its 

 forward prolongation — both of them palingenetic hypoblast — 

 thus contribute material to the formation of the dorsal wall of 

 the intestine, in a region where the underlying cenogenetic 

 hypoblast-cells soon become indistinguishable — a fact in which 

 nearly all observers concur, although they may offer different 

 interpretations. 



When this phenomenon is accomplished the effects of the 

 precocious segregation above sketched have been eliminated, 

 and comparison with lower vertebrates is much easier. Those 

 portions of hypoblast that are enclosed inside the annular 

 '' Mesoblast-hofj" and that have not been sacrificed to the 

 fusion here alluded to, are in their turn utilised in the forma- 

 tion of the dorsal enteric wall : they lose their flattened aspect 

 of the early stages, and become indistinguishable from the rest. 



The fusion between palingenetic and cenogenetic hypoblast 

 having thus finally come about, we can commence our com- 

 parison of the lowest vertebrates and of the mammals with 

 respect to the further processes. 



To begin with the notochord, we may say that its formation 

 out of a continuous strip of mediodorsal hypoblast is also met 

 with in mammals, but that here the strip has been divided 

 into an anterior and a posterior portion, the former (proto- 

 chordal plate) belonging to the group of hypoblast-cells that 

 have undergone cenogenetic displacement, the latter (proto- 

 chordal wedge) forming part of the palingenetic hypoblast and 

 actually growing forwards from the blastopore, and connected 

 with the epiblast in the front wall of the neurenteric canal, as 

 we see in Amphioxus. The mesoblast in Amphioxus arises to 

 the right and left of this strip of protochordal hypoblast in the 

 form of separate diverticula, the walls of which are directly 

 continuous with it. 



In such mammals as the shrew, notwithstanding the very 



