572 ADAM SEDGWIOK. 
In Scyllium and Raja the hyobranchial cleft is formed 
before the spiracular cleft. 
It is interesting to notice in this series of heads the manner 
in which the at first straight mandibular arch is bent upon 
itself at the point which will become the point of articulation 
of the upper and lower jaws. The part anterior to the 
angle develops a forward projection and forms the upper 
jaw—the part behind is bent ventralwards and outwards 
and forms the lower jaw. The widening and shortening of 
the mouth seems largely due to this bending of the mandibular 
arch (cf. series of figures of heads in ventral and side view). 
The view that the mouth is derived from the anterior end of 
the blastopore was originally put forward in my paper on “ The 
Origin of Metameric Segmentation” (‘Quart. Journ. Micr. 
Sci.,’ 1884). Considering the early stage at which the anterior 
end of the blastopore closes in Vertebrates, and the relatively late 
appearance of the mouth, one would not expect to find any 
direct embryological evidence in support of this view. For 
the argument and indirect evidence in favour of it I refer the 
reader to pp. 73 et seq. of my paper above mentioned. To 
that evidence I now add the long slit-like form of the pri- 
mitive Elasmobranch mouth. 
38. Segmentation of the Mesoderm and Development 
of Nerves. 
V. Wyhe! describes the cranial mesoderm in Scyllium as 
segmenting from behind forwards, and he says that in Stage I 
—and not before—the whole of the cephalic mesoderm is 
broken up into somites, and that all these somites contain a 
cavity except the first. 
Kastschenko’ says that the first somite is formed at what 
appears to be the junction of the head and trunk, and that the 
segmentation of the mesoderm extends backwards and forwards 
from this point. Anteriorly it becomes more and more 
1 «Ueber die Mesodermsegmente u. d. Entwick. d. Nervend. Selachier- 
koppes,’ Amsterdam, 1882. 
* * Anat. Anzeiger,’ vol, ili, p. 462, 
