650 EARLY DEVELOPMENT OF THE LACERTILIA. 



I conclude this paper with a concise statement of what 

 appears to me the probable nature of the much-disputed organ, 

 the primitive streak, and of the arguments in support of my 

 view. 



In a paper on the primitive streak in the Quart. Journ. of Mic. 

 Sri., in 1873 (p. 280) [This edition, p. 45], I made the following 

 statement with reference to this subject : "It is clear, therefore, 

 that the primitive groove must be the rudiment of some ancestral 



feature It is just possible that it is the last trace of that 



involution of the epiblast by which the hypoblast is formed in 

 most of the lower animals." 



At a later period, in July, 1876, after studying the develop- 

 ment of Elasmobranch fishes, I enlarged the hypothesis in a 

 review of the first part of Prof. Kolliker's Entwickhings- 

 gcschic/ite. The following is the passage in which I speak 

 of it 1 : 



" In treating of the exact relation of the primitive groove to 

 the formation of the embryo, Professor Kolliker gives it as his 

 view that though the head of the embryo is formed independently 

 of the primitive groove, and only secondarily unites with this, 

 yet that the remainder of the body is without doubt derived 

 from the primitive groove. With this conclusion we cannot 

 agree, and the very descriptions of Professor Kolliker appear to 

 us to demonstrate the untenable nature of his results. We be- 

 lieve that the front end of the primitive groove at first occupies 

 the position eventually filled by about the third pair of proto- 

 vertebrae, but that as the protovertebrae are successively formed, 

 and the body of the embryo grows in length, the primitive groove 

 is carried further and further back, so as always to be situated 

 immediately behind the embryo. As Professor Kolliker himself 

 has shewn it may still be seen in this position even later than 

 the fortieth hour of incubation. 



"Throughout the whole period of its existence it retains a 

 character which at once distinguishes it in sections from the 

 medullary groove. 



" Beneath it the epiblast and mesoblast are always fused, 

 though they are always separate elsewhere ; this fact, which was 



1 Journal of Anat. and Phys., Vol. X. pp. 790 and 791. Compare also my 

 Monograph an Elasmobranch Fishes, note on p. 68 [This edition, p. 281]. 



