428 F. M. BALFOUR. 



ing to Hensen's account, fused together. This, however, is 

 denied by KoUiker, who states that in Mammals, as in Birds, only 

 the epiblast and mesoblast fuse together. Our hypothesis as to 

 the origin of the primitive groove appears to explain in a fairly 

 satisfactory manner all the peculiarities of this very enigmatical 

 organ ; it also relieves us from the necessity of accepting Professor 

 KoUiker's explanation of the development of the mesoblast, though 

 it does not, of course, render that explanation in any way 

 untenable." 



At a somewhat later period Kauber arrived at a more or less 

 similar conclusion, which, however, he mixes up with a number 

 of opinions from which I am compelled altogether to dissent. i 



The general correctness of my view, as explained in my second 

 quotation, appears to me completely estabhshed by Gasser's beau- 

 tiful researches on the early development of the chick and goose," 

 and by my own observations just recorded on the lizard. 

 While at the same time the parallel between the blastopore 

 of Elasmobrancliii and of the Sauropsida, is rendered more com- 

 plete by the discovery of the neurenteric passage in the latter 

 group, which was first of all made by Gasser. 



The following paragraphs contain a detailed attempt to establish 

 the above view by a careful comparison of the primitive streak 

 and its adjuncts in the amniotic vertebrates with the blastopore 

 in Elasmobrancliii. 



In Elasmobranchii the blastopore consists of the following 

 parts : — (1), a section at the end of the medullary plate, which 

 becomes converted into the neurenteric canal ;^ (2), a section 

 forming what may be called the yolk blastopore, which even- 

 tually constitutes a linear streak connecting the embryo with the 

 edge of the blastoderm {?nde my monograph on Elasmobranch 

 fishes, pp. 68 and 81). In order to establish my hypothesis 

 on the nature of the primitive streak, it is necessary to find the 

 representatives of both these parts in the primitive streak of the 

 amniotic vertebrates. The first section ought to appear as a 

 passage from the neural to the enteric side of the blastoderm 

 at the posterior end of the medullary plate. At its front 

 edge the epiblast and hypoblast should be continuous, as they 

 are at the hind end of the embryo in Elasmobranchii, and, 

 finally, the passage should, on the closure of the medullary 

 groove, become converted into the neurenteric canal. All these 

 conditions are exactly fulfilled by the opening at the front end of 



^ " Primitivrinne u. Urmuxd," ' Morphologisches Jahrbuch.,' Band ii, p. 

 551. 



- Gasser, 'Der Primitivstreifen bei Vogelembryoneu,' Marburg, 1878. 



' I use this term for the canal connecting the neural and alimentary 

 tract, which was first discovered by Kowalevskj. 



