BARE FORM OP TITB BLASTODERM OP THE CIIIOK. o81 



been put forward, which may be conveniently distinguished as 

 the differentiation theory and the concrescence theory. 

 His and Rauber are the chief exponents of the theory of concres- 

 cence, and appear to have arrived at similar conclusions inde- 

 pendently of each other. His developed his view in connection 

 with investigations on the embryology of Osseous Fishes, and 

 shortly afterwards extended it to the Elasmobranch and, with 

 some reserve, to the Chick. With reference to the Salmon, 

 His announced his conclusion in the following words : 



" The structural basis (Uranlage) of the body, then, is a 

 flat ring, which has its maximum width and thickness at the 

 future head end, its minimum at the opposite tail end. The 

 two lateral halves of the ring approach and unite retrogressively 

 [successiv] as symmetrical body-halves" (No. 20, pp. 19, 20). 

 In opposition to Oellacher, His maintains that the head end is 

 to be regarded as a fixed point, and that the axial concrescence 

 of the two halves of the ring begins at this end and advances 

 towards the tail end, which is thus the last formed part of 

 the embryo. The entire ring is thus brought together along 

 the axial line, and this process goes hand in hand with the 

 epibolic expansion of the blastoderm. 



The Elasmobranch embryo is formed in a similar manner, 

 but with this difference, that only a posterior portion of the 

 ring takes a direct part in forming the embryo, so that the pro- 

 cess of building np the embryo is completed long before the 

 final closing in of the yolk by the blastoderm. 



In his excellent memoir, entitled / Primitive Streak and 

 Neurula of the Vertebrates,' Hauber has given a brief survey 

 of vertebrate embryology, and summarised the more important 

 results reached by others and himself, with a view to making 

 clear their theoretical bearings. Rauber claims that the Avian 

 embryo arises, in the main, by a longitudinal concrescence of 

 the two halves of the germinal ring (" Keimring"), in the 

 same manner as the Piscian embryo ; and further, that this 

 process underlies the formation of all vertebrate embryos. 



In the case of the Chick, both His and Rauber have called 

 attention to a lunula-shaped area that appears, after about six 



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