ORIGIN AND STRUCTURE OF THE HEAD 147 



vertebra '). Besides the prostomium it contains five segments, 

 two pro-otic and three post-otic. No post-branchial segments, 

 i.e. segments behind the last gill-slit, have been incorporated 

 into the skull, the occipital arch being situated over the last 

 (fifth) gill-slit (Miss Platt, 1898, p. 452). As a consequence of 

 the restricted number of post-otic head segments the sphere of 

 influence of the vagus extends beyond the cranio-vertebral limit 

 and causes the first (in Anurans even the second) free spinal 

 nerve to lose its dorsal root and ganglion. The vagus never sup- 

 plies more than three gill-slits and as a consequence contains 

 the rami post- and praetrematici of only two spinal nerves 

 behind it, being the "spinalartiger Vagusanhang" and the 

 first free spinal nerve, to which the dorsal root is wanting. 



The hypoglossus-musculature, according to Miss PLATT 

 (1898, p. 452), is produced in Urodelans by ventral buds 

 from the last epibranchial myotome (the third post-otic somite) 

 and by two postbranchial ones (4*'' and 5^^ post-otic somites) 

 and is innervated by the ventral roots belonging to the 

 latter two, being the first two free spinal nerves which 

 constitute the wholly postcranial cervical plexus, or hypo- 

 glossus. There is no epibranchial musculature in Amphibians. 



Head of Selachians. — In Selachians and Sauropsids, 

 finally, the first processes of development are in a corres- 

 ponding way influenced by the enormous accumulation of 

 yolk in the egg. In both, the metencephalon, in contrast 

 with both the foregoing groups, has strongly developed, and 

 the hypophysis no longer originates in front of the mouth 

 involution but from its roof. In both groups new segments 

 have been added to the skull which we shall accordingly call 

 a neocranium. Whereas in the glossopharyngeus-somite 

 no myotome and in the primary vagus segment only a very 

 rudimentary myotome develops, we find in Selachians 

 behind the primary vagus some four well-developed myotomes 

 belonging to the head region and instead of one neural 

 arch the rudiments of four have been described for Acanthias 

 by HOFFMANN (1894, p. 638), of which the anterior one 

 has been compared by Sewertzoff (1895, p. 260) with the 

 occipital arch of Amphibians. In front of it, just as in 



^) If, at least, we do not consider the rudimentary "prae-occipital 

 arch", sometimes found in Urodelans in front of the "occipital arch", 

 and to which the occipital arch of Anurans, according to Van Seters 

 (1921), corresponds, as another vertebra. 



