43G waltl:[i he ape. 



(No. 2G), in believing that the primitive streak mesoblast sup- 

 plies the whole of the embryonic area. 



With regard to this point my results are in entire agreement 

 v/ith those of Balfour anil Deighton, expressed in their account 

 of the development of the chick (No. 2), who consider that the 

 anterior portion of the mesoblast is derived as two lateral 

 plates from the hypoblast, while the axial hypoblast gives rise 

 to the notochord. 



Further, the similarity of the origin of the epiblast, hypo- 

 blast, and mesoblast of the embryo and of the notochord in the 

 mammal is so strikingly similar to the relations of the same 

 organs in Amj)liioxus that Kolliker's (loc. cit.) statements as to 

 the dissimilarity of the germinal layers of mammals with those 

 of other animals appears to me to require some modification ; 

 and RepiachofF's recently expressed opinions (No. 24) that 

 there is no homology between the germinal layers of higher 

 Vertebrata and Amphioxus, receives no sui)port from what is 

 known of mammalian embryology. 



Finally, it is very generally believed that mammals are 

 descended from animals which possessed a large yolk sac, and 

 it is stated that the blastodermic vesicle is a remnant of this 

 yolk sac. If this be true (and as far as we know there seems 

 to me to be no reason to doubt it), the ])rimitive streak of 

 mammals is homologous with the same structure in birds, and 

 the existence of such an arrangement, together with the 

 presence of a complete neurenteric canal (which I shall 

 describe later) in the mammal, is aucther instance of the mor- 

 phological facts which led Balfour (No. 1) to conclude that the 

 primitive streak was homologous with the true vertebrate 

 blastopore. 



The views as to the relations of the layers at the front end 

 of the primitive streak will be more advantageously noticed in 

 the following section. 



