EARLY DEVELOPMENT OF THE LACERTILIA. 653 



more complete 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 primi- 

 tive streak and its adjuncts in the amniotic vertebrates with the 

 blastopore in Elasmobranchii. 



In Elasmobranchii the blastopore consists of the following 

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

 becomes converted into the neurenteric canal 1 ; (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 (vide monograph on Elasmobranch 

 fishes, pp. 281 and 296). 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 con- 

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

 the primitive streak of the lizard (vide woodcut, fig. I, p. 647). 

 In the chick there is at first no such opening, but, as I hope to 

 shew in a future paper, it is replaced by the epiblast and hypo- 

 blast falling into one another at the front end of the primitive 

 streak. At a later period, as has been shewn by Gasser 2 , there 

 is a distinct rudiment of the neurenteric canal in the chick, and a 

 complete canal in the goose. Finally, in mammals, as has been 

 shewn by Schaffer 3 for the guinea-pig, there is at the front end 

 of the primitive streak a complete continuity between epiblast 

 and hypoblast. The continuity of the epiblast and hypoblast at 

 the hind end of the embryo in the bird and the mammal is a 



1 I use this term for the canal connecting the neural and alimentary tract, which 

 was first discovered by Kowalevsky. 



2 Loc. cit. 



3 " A contribution to the history of the development in the Guinea-pig," Journal 

 of Anat. and Phys. Vol. xi. pp. 332 336. 



