146 THE ANCESTRY OF VERTEBRATES 



myzon, there are three, as found by Miss Platt (1897, 

 p. 850) in Necturus. To judge from MARCUS' (1910) figures 

 and descriptions this also holds good for Gymnophiones (the 

 last, permanent, myotome of the head e.g. is situated here also 

 above the space between the 4**' and the 5**^ visceral cleft), 

 although certain corrections of MARCUS' interpretations seem 

 necessary. In the -axolotl (and the sturgeon) Sewertzoff 

 (1895, p. 260) observed only two post-otic somites in front 

 of the occipital arch but, since in Selachians their number 

 is also three, he leaves open the possibility that one has 

 fallen out behind the auditory capsule, while Miss PLATT 

 (1898, p. 450) suggests that SEWERTZOFF has made amis- 

 take. This has been fully confirmed by GOODRICH (1911, 

 p. 116) who also comes to the conclusion that there 

 are three post-otic segments included in the skull of 

 Siredon. This number corresponds to that found in Petromyzon 

 between the auditory capsule and the first neural arch. 



Of the three post-otic head somites the anterior two, the 

 glossopharyngeus and the primary vagus somite, no longer 

 develop permanent muscles, they have been crushed out of 

 existence, so to speak, by the auditory capsule. A little 

 transient myotome, designated by him as y, is found by 

 Marcus (19 id, p. 430) close to the vagus-ganglion and also a 

 corresponding ventral rooiy. Afterwards, however, both break 

 down while in Necturus, according to Miss PLATT (1898, 

 p. 447), and in Siredon, according to GOODRICH (1911), a 

 few fibres persist, continuous with the dorsal part of the 

 third post-otic somite. This somite, being the one of the 

 "spinalartiger Vagusanhang", produces in Urodelans the 

 anterior segment of the trunk musculature, attached, as 

 recorded above, at the auditory capsule and inneivated by 

 the ventral occipital nerve (wrongly) designated as z. 



Recently my friend VAN Seters (1921) has come to the 

 interesting conclusion, that in Anurans the skull is sti'l 

 one segment shorter than in Urodelans. Thus the first 

 post- cranial segment would be the one corresponding to the 

 "spinalartiger Vagusanhang." This explains why in Anurans, 

 where the myotome belonging to this segment, together 

 with its ventral nerve-root, atrophies, not only the first 

 (the "spinalartiger Vagusanhang") but also the second spinal 

 ganglion, gets lost. 



We shall call the cranium of Urodelans a complete 

 p a 1 a e o-c r a n i u m, comprising the rudiment of only one 



