568 STRUCTURE OF VERTEBRATES 



of the nineteenth century showed, however, that the mesoblast 

 of the head is originally segmented nearly to its tip, and that the 

 obhteration of segmentation in the adult is merely the result of 

 the development of the sense organs and of the formation of 

 jaws. Further work has confirmed and extended this interpre- 

 tation, but it is still not possible to give a fully satisfactory 

 description of the cranium in terms of segmentation. 



The embryology of the mesoblast described for the chick on 

 p. 649, and Fig. 512, holds pretty generally throughout the verte- 

 brates. The dorsal part is segmented, and each somite is early 



Posterior I cm it of head. 



VesttgLai 

 V Spinal. 



'Z^'Spinai. 



Fig. 445. — Diagram showing the segmentation of the head in a selachian. — 



Simphfied from Goodrich. 

 A-F, Gill slits ; i-ii, somites ; HI-X, cranial nerves. 



divided into a myotome, forming muscles, and a sclerotome, 

 forming skeletal elements. The main nerves are not themselves 

 segmented, but since they emerge between the vertebrae and 

 supply the muscles, they have a segmental arrangement impressed 

 on them ; the ventral roots supply the myotomes, and the dorsal 

 roots, chiefly but not entirely sensory, run behind the myotomes. 

 The ventral part of the mesoblast remains unsegmented as the 

 lateral plate. In the head of the embryo Scyllium a similar 

 arrangement holds as far forward as the anterior end of the 

 notochord — that is, to the level of the future hypophysis. In 

 the region which will become the future head there are eight 

 pairs of hollow somites, here called head cavities, which form 



