1012 



The gill-pouches, five in number in yecturus, the foremost corre- 

 sponding to the spiracle, again regularly alternate with the 6 anterior 

 somites (Platt, 1894, 1897). Thus the five head somites are all 

 epibranchial, the occipital arch lying right above the last gill-slit 

 (Platt, 1897, p. 452). By the restricted backward extension of the 

 cranium the sphere of influence of the vagus causing the suppres- 

 sion of the subsequent spinal ganglia extends beyond the cranio- 

 vertebral limit; in Amphibians as a consequence the first free spinal 

 ganglion is always absent. Since in phaneroglossan An urans also 

 the ventral root gets lost (see above), the first spinal nerve is wholly 

 absent here. The vagus does not supply more than three gill-slits, 

 being that of the primary vagus, of the "spinalarfiger Vagusanhang" 

 and the first free spinal nerve which either has no dorsal root or 

 is wholly absent. 



The hypobranchial musculature is formed in Urodelans from 

 ventral buds of the anterior two post-branchial (4'^ and 5''i post- 

 otic somite) and the last epibranchial myotome (3*^ post-otic somite) 

 (Miss Platt, 1897, p. 452) and innervated by the ventral roots of 

 both the former somites, being the first two free spinal nerves, 

 which in Urodelans together constitute the wholly post-cranial 

 hypoglossus. The latter now partly lies within the sphere of inflirence 

 of the vagus which causes the anterior root to have no dorsal 

 ganglion. The ventral root of the last epibranchial myotome gets lost, 

 as was mentioned above. 



Epibranchial musculature is absent. 



The olfactory grooves, situated in Annelids dorsally on the pro- 

 stomium just in front of the limit between prostomium and first 

 segment, are found in Craniotes at the corresponding place, viz. 

 ventrally on the prostomium just in front of the mouth. Only in 

 Petromyzon has a secondary monorhinism been established. 



In Selachians as well as in Amniotes the enormous yolk-contents 

 of the egg has in a corresponding way influenced the course of the 

 earliest development, in both these the metencephalon has developed 

 and the pituitary body does not originate any longer in front, 

 but from the roof of the mouth evolution. In both the skull has 

 annexed a number of vertebral elements constituting the regio occipi- 

 talis. In Acanthias the rudiments (or at least the indication, Sewert- 

 zoFF, 1899) of four vertebrae may be observed (Hoffmann, 1894, 

 p. 638), the foremost of which corresponds to the occipital arch of 

 Amphibians (Skwertzoff, 1895, p. 260) and, like the latter, is 

 separated by three somites from the auditoi-y capsule. Thns three 

 segments have been added to the skull; in Scyllium and Pristiurus 



