NOTES ON ELASMOBRANCH DEVELOPMENT. 2^9 



here must be regarded as meaning the anterior end of the 

 body, for it is not possible in these young embryos to dis- 

 tinguish the head from the trunk. I am, however, in entire 

 agreement with the statement that there is a stage in which 

 there is a considerable tract of mesoderm in front of the first 

 formed somite, which is entirely unsegmented, and with no signs 

 of differentiation into somatic and splanchnic layers. But in 

 Pristiurus this stage is of very short duration, for, according to 

 Balfour, even in Stage D there is a cavity in the anterior part 

 of the mesoderm. I can entirely confirm Balfour as to the 

 presence of this cavity at this early age in Pristiurus ; but it is 

 not, as he seems to imply, ever continuous with the general 

 body-cavity. It is, indeed, a somite — the second or mandi- 

 bular somite of v. Wyhe, — and its appearance is followed by 

 the breaking up of the mesoderm between it and the first 

 so-called trunk somite into successive and contiguous but 

 indistinct somites. I am not able to say in what order these 

 somites are formed, whether from behind forwards, as 

 Kastschenko maintains, or in the reverse direction. All I can 

 say on this subject is that in Pristiurus the mandibular somite 

 is formed before those behind it, and that in Scyllium I have 

 an embryo a little older than Stage F, but younger consider- 

 ably than Stage G, in which the whole of the mesoderm 

 in front of the first so-called trunk somite is broken up into 

 somites successively traceable in a series of transverse sections. 

 The first of these somites (the second of Wyhe) is the most 

 distinct and, I expect, the first formed, as in Pristiurus. 



This early segmentation of the anterior part of the meso- 

 derm into somites almost exactly like those in the hinder part 

 of the body is a morphological point of great interest. It is 

 very transitory in the genera mentioned, and disappears before 

 any trace of the pharyngeal pouches are formed, except in the 

 case of the mandibular somite, and possibly also of the one 

 next it. In Stage I, where, according to v. Wyhe, the segmen- 

 tation of the anterior part of the mesoderm is complete, I 

 cannot find in either Scyllium or Pristiurus or Raja any 

 of the somites described by him as the fourth, fifth, and sixth ; 



VOL. V. PAKT II. J-0 



