574 ADAM SEDGWICK. 
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 continuons 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 reversedirection. 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 ; 
