12 SKULL 



come to pass through the cranial wall — so-called spino-occipitals. 

 The more anterior ventral roots only represent the hypoglossal 

 nerve, already described, p. 11, but more posterior and normally 

 developed spinal nerves may also be included, the ' occipito-spinals,' 

 which supply the anterior trunk, myotomes, etc. (Jackson and 

 Clarke [236], Gegenbaur [1G0], and especially Furbringer's admirable 

 monograph [143]). 



The exact early history of the skull is sunk in the obscurity of 

 the past, and may perhaps never be reconstructed from embryo- 



B 



s. -A 



nt. 



"s. 



-cc 



csr 



Fig. 



Diagrams illustrating the development of the skull in the Craniata. A, early stage ; B, later 



age. a, auditory vesicle; ".<•, auditory capsule; e, ethmoid region ; n, nasal sac ; m.c, nasal 



capsule; nt, notochord ; o, optic vesicle; o.c, optic capsule ; o.s, occipital segment or sclerotome ; 



p, parachordal region : s, vertebral sclerotome ; t, trabecular region. The dotted areas represent 



cartilage and precartilage. 



logical data. That the occipital region is segmental is clear. 

 The parachordals themselves show but slight indications of sub- 

 division (Piatt, in Urodela [331a]); the trabeculae show none 

 whatever. How far segments in the prechordal and even in the 

 parachordal region of the head may have been modified and 

 obliterated before cartilage developed — how far, in other words, 

 the anterior region was ' cephalised ' before the skull arose — 

 in the ancestors of modern Craniates remains an unsolved 

 problem. 



But it must be remembered that the infundibulum, to which the 

 notochord always reaches in Craniates, probably lies at or near the 



