248 ADAM SEDGWIOK. 



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 Scy Ilium 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 

 diflPerent genera of Elasmobranchs differ, as I hope to show, 

 very remarkably in the condition of the mesoderm 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 supplementingit, 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 protovertebrse." This statement I can 

 entirely confirm for Pristiurus and Scylliura; 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 protovertebrse." The term head 



