BAEB EOEM OF THE BLASTODERM OE THE CHICK. 383 



question, and stated at some length his objections to the con- 

 crescence theory. Balfour takes the Elasmobranch embryo as 

 a type, a case which His regards as the best illustration of his 

 own view. " The Elasmobranch embryo/' says Balfour, 

 " arises from a differentiation of the edge of the blastoderm, 

 which extends inwards from the edge for some little distance. 

 This differentiation is supposed to contain within 

 itself the rudiments of the whole embryo, with the ex- 

 ception of the yolk-sack ; and the hinder extremity of it, at the 

 edge of the blastoderm, is regarded as corresponding with the 

 hind end of the body of the adult. The growth in length 

 takes place by a process of intussusception, and till there are 

 formed the full number of mesoblastic somites it is effected, as 

 in Chsetopods, by the continual addition of fresh somites 

 between the last-formed somite and the hind end of the body" 

 (p. 254). 



The "His-Rauber view" is introduced as "a somewhat 

 paradoxical view," supported by three not very forcible argu- 

 ments, and a number of minor arguments not worth mention- 

 ing. These three arguments are stated to be — (1) the continuity 

 between the thickened edge of the blastoderm and the medul- 

 lary folds ; (2) the embryometrical investigations of His ; and 

 (3) some of the phenomena of double monsters studied by 

 Rauber. Balfour says that the first argument affords no sup- 

 port for either theory, and the second appears to prove his own 

 theory of growth. Kauber's view of " pluriradial development" 

 is passed over without a word of comment, presumably because 

 it was supposed to be without special importance to the 

 discussion. 



Balfour's objections to the concrescence theory may be sum- 

 marised as follows : 



1. The medullary groove closes behind earlier than in front 

 and the groove does not terminate behind in an acute angle, as 

 might be expected if the embryo were formed by the coales- 

 cence of the edges of the blastoderm. 



2. The formation of the neurenteric canal would make it 

 impossible for any further increase in length by concrescence. 



