26 f . M. BALFOUR. 



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. Sci./ in 1873 (p. 280), 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 re- 

 view of the first part of Prof. KoUiker's ' Entwicklungsgeschichte.^ 

 The following is the passage in which I speak of it '} 



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

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

 tliat 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. TVith this conclusion we cannot 

 agree, and the very descriptions of Professor Kolliker appear to 

 us to demonstrate the untenable nature of his results. We 

 believe that the front end of the primitive groove at first occu- 

 pies the position eventually filled by about the third pair of 

 protovertebrse, but that as the protovertebree are successively 

 formed, and the body of the embryo grows in length, the primi- 

 tive groove is carried further and further back, so as always to 

 be situated immediately behind the embryo. As Professor Kol- 

 liker himself has shown 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 j 

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

 originally shown by ourselves, has been very clearly brought out 

 by Professor KoUiker's observations. 



" The features of the primitive groove which throw special 

 light on its meaning are the following : 



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

 embryo. 



'' (2.) The epiblast and mesoblast always become fused 

 beneath it. 



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



^ * Journal of Anat. and Phys./ vol. x, pp. 790 and 791. Compare also 

 my monograph on ' Elasmobranch Fishes,' note on p. 68. 



