EARLY DEVELOPMENT OF THE LACERTILIA. 651 



originally shewn by ourselves, has been very clearly brought out 

 by Professor Kolliker's observations. 



" The features of the primitive groove which throw special 

 light on its meaning are the following : 



"(i) It does not enter directly into the formation of the 

 embryo. 



" (2) The epiblast and mesoblast always become fused be- 

 neath it. 



" (3) It is situated immediately behind the embryo. 



" Professor Kolliker does not enter into any speculations as to 

 the meaning of the primitive groove, but the above-mentioned 

 facts appear to us clearly to prove that the primitive groove is a 

 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 in- 

 vestigations 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 

 embryo, 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 



42 2 



