652 EARLY DEVELOPMENT OF THE LACERTILIA. 



situation of the primitive groove immediately behind the em- 

 bryo, as well as the fact of its not becoming converted into any 

 embryonic organ would be explained. The central groove might 

 probably also be viewed as the groove naturally left between 

 the coalescing edges of the blastoderm. 



"Would the fusion of epiblast and mesoblast also receive its 

 explanation on this hypothesis ? We are of opinion that it 

 would. At the edge of the blastoderm which represents the 

 blastopore mouth of Amphioxus all the layers become fused 

 together in the anamniotic vertebrates. So that if the primitive 

 groove is in reality a rudiment of the coalesced edges of the 

 blastoderm, we might naturally expect the layers to be fused 

 there, and the difficulty presented by the present condition of 

 the primitive groove would rather be that the hypoblast is not 

 fused with the other layers than that the mesoblast is indis- 

 solubly united with the epiblast. The fact that the hypoblast is 

 not fused with the other layers does not appear to us to be fatal 

 to our hypothesis, and in Mammalia, where the primitive and 

 medullary grooves present precisely the same relations as in 

 birds, all three layers are, according to Hensen's account, fused 

 together. This, however, is denied by Kolliker, 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 Kolliker'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 Rauber 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 1 . 



The general correctness of my view, as explained in my 

 second quotation, appears to me completely established by 

 Gasser's beautiful researches on the early development of the 

 chick and goose 2 , and by my own observations just recorded on 

 the lizard. While at the same time the parallel between the 

 blastopore of Elasmobranchii and of the Sauropsida, is rendered 



1 " Primitivrinne u. Urmund," Morphologisches Jahrbuch, Band II. p. 551. 



2 Gasser, Der Primitivstreifen bei Vogelembryonen, Marburg, 1878. 



