NOTES ON ELASMOBRANCH DEVELOPMENT. 579 
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 frontend. In Pristiurus 
and Raja 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) ; (2) 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 
