EARLY DEVELOPMENT OF THE LACERTILIA. 427 



rudimentary structure, the origin of which can only be com- 

 pletely elucidated by a knowledge of the development of the 

 Avian ancestors. 



" In comparing the blastoderm of a bird with that of any 

 anamniotic vertebrate, we are met at the threshold of our inves - 

 tigations by a remarkable difference between the two. Whereas 

 in all the lower vertebrates the embryo is situated at the edge 

 of the blastoderm, it is in birds and mammals situated in the 

 centre. This difference of position at once suggests the view 

 that the primitive groove may be in some way connected with the 

 change of position in the blastoderm which the ancestors of 

 birds must have undergone. If we carry our investigations 

 amongst the lower vertebrates a little further, we find that the 

 Elasmobranch embryo occupies at first the normal position at 

 the edge of the blastoderm, but that in the course of develop- 

 ment the blastoderm grows round the yolk far more slowly in 

 the region of the embryo than elsewhere. Owing to this, the 

 embryo becomes left in a bay, the two sides of which eventually 

 meet and coalesce in a linear fashion immediately behind the em- 

 bryo, thus removing the embryo from the edge of the blasto- 

 derm and forming behind it a linear streak not unlike the primi- 

 tive streak. We would suggest the hypothesis that the primitive 

 groove is a rudiment which gives the last indication of a change 

 made by the Avian ancestors in their position in the blastoderm, 

 like that made by Elasmobranch embryos when removed from the 

 edge of the blastoderm and placed in a central situation similar 

 to that of the embryo bird. On this hypothesis the situation of 

 the primitive groove immediately behind the embryo, 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 ex- 

 planation 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 auaui- 

 niotic 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 indissolubly united with the epi- 

 blast. 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 pre- 

 cisely the same relations as in birds, all three layers are, accord- 



