EARLY DEVELOPMENT OV THE LACERTILIA. 429 



the primitive streak of the lizard [vide woodcut, fig. 1). In 

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

 show 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 shown by Gasser/ 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 

 shown by Schafer^ 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 rudiment of the continuity of these layers at the dorsal lip of 

 the blastopore in Elasmobranchii, Amphibia, &c. The second 

 section of the blastopore in Elasmobranchii or yolk blastopore is, 

 I believe, partly represented by the primitive streak. The yolk 

 blastopore in Elasmobranchii is the part of the blastopore belong- 

 ing to the yolk sac as opposed to that belonging to the embryo, 

 and it is clear that the primitive streak cannot correspond to the 

 whole of this, since the primitive streak is far removed from the 

 edge of the blastoderm long before the yolk is completely enclosed. 

 Leaving this out of consideration the primitive streak, in order 

 that the above comparison may hold good, should satisfy the 

 following conditions : 



1. It should connect the embryo with the edge of the blasto- 

 derm. 



2. It should be constituted as if formed of the fused edges of 

 the blastoderm. 



3. The epiblast of it should eventually not form part of the 

 medullary plate of the embryo, but be folded over on to the 

 ventral side. 



The first of these conditions is only partially fulfilled, but, con- 

 sidering the rudimentary condition of the whole structure, no 

 great stress can, it seems to me, be laid on this fact. 



The second condition seems to me very completely satisfied. 

 Where the two edges of the blastoderm become united we should 

 expect to find a complete fusion of the layers such as takes place 

 in the primitive streak ; and the fact that in the primitive streak 

 the hypoblast does not so distinctly coalesce with the mesoblast 

 as the mesoblast with the epiblast cannot be urged as a serious 

 argument against me. 



The growth outwards of the mesoblast from the axis of the 

 primitive streak is probably a remnant of the invagination of the 



1 Loc. cit. 



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

 'Journal of Anat. and Phys.,' vol. xi, pp. 333—336. 



