NOTES ON ELASMOBRANCH DEVELOPMENT. 247 



In Scy Ilium 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, and these 'Studies/ vol. ii). Considering the early 

 stage at which the anterior eud of the blastopore closes in Verte- 

 brates, 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 primitive Elasmobranch mouth. 



3. Segmentation of the Mesoderm an d Development 



of Nerves. 



V. Wyhe^ describes the cranial mesoderm in Scyllium as 

 segmenting from behind forwards, and he says that in Stage 1 

 — 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 



' ' Ueber die Mesodermsegmente u. d. Entwick. d. Nerven d. Selacliier- 

 koppes,' Amsterdam, 1882. 



^ ' Anat. Anzeiger,' vol. iii, p. 462. 



