NOTES ON ELASMOBRANCH DEVELOPMENT. 573 
indistinct as the front end of the embryo is approached, so 
that the anterior part of the cephalic mesoderm is at no stage 
of development broken up into somites. This unsegmented part 
of the cephalic mesoderm, which corresponds, according to 
Kastschenko, to several somites, is comprised in the second 
somite of Wyhe. The first somite of Wyhe occupies, in 
Kastschenko’s opinion, a special position. Kastschenko’s 
observations were made on the genera Scyllium and 
Pristiurus, but he does not state precisely the ages of the 
embryos to which his observations refer, nor distinguish 
between the genera in describing his observations. As the 
different genera of Elasmobranchs differ, as I hope to show, 
very remarkably in the condition of the mesoderm and during 
these early stages, this latter point is one of considerable 
importance. 
It is perfectly obvious to anyone who examines Elasmo- 
branch development that the work of these two observers 
has been exceptionally thoroughly and carefully done; and if 
the results and views which I have arrived at differ from theirs, 
I would wish my work to be considered alongside of theirs, not 
as contradicting, but as supplementing it, by the future workers 
who succeed in obtaining a fuller and more accurate knowledge 
of the development of the different genera of this interesting 
group. 
Balfour (‘ Elasmobranch Fishes,’ Mem. Ed., p. 302), in de- 
scribing Pristiurus, says that “ coincidently with the appear- 
ance of a differentiation into a somatic and splanchnic layer 
the mesoblast plates become partially split by a series of 
transverse lines into protovertebre.”’ This statement I can 
entirely confirm for Pristiurus and Scyllium; its import- 
ance has not been fully appreciated or understood. What it 
means is this, that the body-cavity at the very first 
sign of its appearance (differentiation of mesoderm into 
somatic and splanchnic layers) is segmented. 
Balfour goes on to say, ‘“‘ In the head, so far as I have yet 
been able to observe, the mesoblastic plates do not at this 
stage (D) become divided into protovertebre.” The term head 
