251 ADAM SEDGWICK. 



There can be no question that the first or preoral somite 

 develops in connection with this solid mass of cells, but 

 whether entirely from it, as Wyhe appears to maintain, or 

 only partly from it, is difficult to say. In Scyllium there are 

 very clear indications that a part of the tissue from which the 

 somite develops is derived from a paired ingrowth from the 

 ectoderm. In Stage G the cell mass is continued forwards on 

 each side in continuity with the ectoderm, and these paired 

 tracts present the appearance of ingrowths. 



The mass of cells of which I am speaking presents very 

 remarkable differences in its relation to adjacent organs in the 

 different genera that I have examined. In Scyllium and 

 Pristiurus it is continuous with the ventral ectoderm 

 throughout its whole extent from the earliest stage at which 

 I have seen it, i. e. Stage F, or the earliest stage at which the 

 ventral ectoderm is folded in. 



In Scyllium it is for the most part not continuous with 

 the medullary ectoderm, unless there is such a continuity, of 

 which I am not certain, at its very front end. In Pristiurus 

 and Eaja it is markedly continuous with the medullary ecto- 

 derm throughout its entire extent, while in Raja the dorsal 

 lateral outgrowths, which are soon formed from it, are also 

 continuous with the medullary ectoderm. Further, Raja 

 differs from the other two genera in that this cell-mass is not 

 continuous with the ventral ectoderm at all (excepting through 

 the endoderm and buccal slits). 



As Wyhe has correctly stated, the first or premandibular 

 somite of Balfour is formed by the hollowing out of this mass 

 of cells and its lateral prolongations, and Kastschenko seems 

 to be justified in placing it in a different category from the 

 other somites. It differs from the other somites in two 

 respects : (1) in its connection at origin with the ectoderm, 

 either of the body-wall or of the neural tube (Raja, Pris- 

 tiurus) ; (3) in its continuity with its fellow across the middle 

 line. 



Before leaving this cell mass which gives rise to the first 

 somite, and which eventually breaks off from the various 



