BARE FORM OF THE BLASTODERM OF THE CHICK. 385 



share the opinion with some others that such an agreement 

 does exist, and in it I find one of the most conclusive evidences 

 of genetic affinity. This agreement consists not only, in the 

 metameric division of the embryo, which is the only point of 

 agreement alluded to by Balfour, but also in the formation of 

 the embryo by concrescence of the two halves of the embryonic 

 ring. 



Accordingly I hold that Balfour has left out of consideration 

 one of the most important elements of (he problem. This is a 

 criticism from a general point of view ; but, as it serves to 

 make clear the standpoint from which I propose to consider the 

 question at issue, it comes properly enough before the considera- 

 tion of the above-named objections, to which we may now 

 pass. 



There is nothing in the first objection which has not been 

 anticipated and answered in a general way in the writings of 

 His and Kauber, and I need not here repeat their statements. 

 It may be worth while, however, to call attention to another 

 way of meeting the supposed difficulty. It is now quite clear 

 that the primitive groove and the medullary groove must not 

 be confounded, and it is equally clear tbat the concrescence 

 theory must be able to account for them both in every instance, 

 in order to maintain itself. But is it possible to keep up the 

 distinction between them in all cases ? What, for example, 

 can be called the primitive groove of the Elasmobranch embryo ? 

 It has been said that here there is no such structure, and by 

 some the medullary groove has been called prinutive groove. 

 There is some danger of confusion on this point, and I believe 

 this confusion underlies the objection we are here considering. 

 If we break loose from all mental pictures suggested by the 

 word groove, and adhere strictly to the definition of the primi- 

 tive groove as the plane of junction of the lips of the 

 blastopore — a definition to which most embryologists will cer- 

 tainly assent — then it follows, on the concrescence theory, that 

 something of this nature must be recognised in the Elasmo- 

 branch embryo. liOoking at the question from this standpoint, 

 it is plain that the medullary groove is not identical with the 



