EARE FORM OF THE BLASTODERM OP THE OHIGK. 395 



If the theory of concrescence is applicable to the vertebrates, 

 it is evident that we cannot regard the two ends of the embryo 

 as equally old portions, which are gradually pushed in opposite 

 directions by the interpolation of fresh somites successively 

 budded off from the hind end. The discovery of two " Pol- 

 zelien des Mesoderms" at the extreme hind end of the embryo 

 of Amphioxus by Hatschek, supports the opinion that the 

 lengthening and the metameric division of the vertebrate em- 

 bryo take place in fundamentally the same manner as in the 

 Annelids. 



The error of the differentiation theory does not lie in the 

 assumption of intussusceptional growth, but in excluding the 

 concomitant process of concrescence. The formation of the 

 vertebrate embryo does not afford such conspicuous evidences 

 of concrescence as that of the Annelid embryo ; but even in the 

 more doubtful cases of the Osseous Fish and the Bird, both His 

 (No. 22) and Kupffer (No. 31) have shown that there is a 

 general movement of the embryoplastic material which must, 

 in my opinion, be regarded as a disguised form of concrescence. 

 No other explanation of the phenomena can claim to bring 

 them into relation with that form of development still pre- 

 served in the nearest invertebrate relatives of the vertebrates. 



According to the concrescence theory, it will not do to 

 regard the primitive streak as analogous to the " linear streak" 

 behind the Elasmobranch embryo, as has been done by Balfour. 

 The primitive streak lies within the germinal ring, and is 

 continuous with it at the hind border ; the " linear streak" is 

 located much further back, being entirely behind the embryo- 

 plastic portion of the ring. Another important difference, not 

 alluded to by Balfour, consists in the fact that the primitive 

 streak arises even before the medullary folds ; while the 

 " linear streak" appears after the embryo is formed, and even 

 after the process of constriction has narrowed the connection 

 of the embryo with the yolk to a slender cord, the umbilical 

 stalk. Nothing analogous to this "linear streak" appears in 

 the normal blastoderm of the Chick. In the exceptional form 

 of the blastoderm which I have described, the streak connect- 



