326 THE MAMMALIAN EMBRYO. [CFAP. 



with the epiblast. The notochord in fact behaves exactly 

 as did the axial hypoblast in the earlier stage. 



The peculiar relations just mentioned are precisely similar to 

 those we have ah-eady described in the chick (p. 60). They 

 receive their explanation by comparison with the lower types. 



The cells which form the junction between the epiblast and 

 the axial hypoblast constitute in the lower types the front wall of 

 a passage perforating the blastoderm and leading from the ex- 

 terior into the alimentary canal. This passage is the vertebrate 

 blastopore. 



In the chick we have seen (p. 72) this passage is present at a 

 certain stage of development as the neurenteric canal ; and in the 

 duck at a still earHer stage. It is also present at an early stage 

 in the mole. 



The presence of this blastopore renders it clear that the blas- 

 topore discovered by Ed. van Beneden cannot have the meaning 

 he assigned to it in comparing it with the blastopore of the 

 frog. 



To recapitulate. At the stage we have now reached 

 the three layers are definitely established. 



The epiblast is derived partly from the outer layer 

 of segmentation spheres and partly from the larger pro- 

 portion of those segmentation spheres which constitute 

 the inner mass. The hypoblast arises from the few 

 remaining cells of the inner mass ; while the mesoblast 

 has its origin partially from the epiblast of the primitive 

 streak and partially from the hypoblast cells anterior to 

 the primitive streak. 



During the period in which these changes have been taking 

 place, the rudiments of a vascular area become formed, and while 

 as KSBiker has shewn, the mesoblast of this portion is to some 

 extent derived from the' mesoblast of the primitive streak, it is 

 possible that a portion of it owes its origin to hypoblastic meso- 

 blast. 



