HEAD OF SQUALUS 461 



somites 3 and 4, glossopharyngeal between 4 and 5. The 

 rudiment of the first branch of the vagus overlies somite 5, 

 which is the most anterior of the post-otic somites to develop 

 muscle-fibres. 



Text-fig. 3 shows clearly the relation of the vagus to the 

 somites. It covers the posterior border of the fifth somite and 

 most of the sixth. By tracing up through later stages I have 

 arrived at the conclusion that it is the sixth somite which forms 

 the first permanent myotome. For the greater part median to 

 the vagus, its posterior dorsal corner is prolonged into a 

 hook-shaped process which, lapping round the posterior edge 

 of the vagus, extends forwards laterally to it. The hook-shaped 

 process can be seen in an incipient condition in this (Text-fig. 3) 

 and in subsequent stages ; likewise that the sixth somite 

 corresponds to the third gill-slit, which is of course related to 

 the second branch of the vagus. There is also serial correspon- 

 dence between spiracle, gill-slits 1 and 2, and the third, fourth, 

 and fifth somites respectively. The estabhshment of this corre- 

 spondence is important, for some authors (Dohrn, Froriep) have 

 described a varyingly large number of somites under the 

 auditory capsule. I am convinced that there is only one somite 

 between the facial and the glossopharyngeal in Squalus. Text- 

 fig. 4 is a drawing of a single section, and the region beneath 

 the auditory capsule from two sections of another embryo is 

 shown in Text-fig. 5 and under higher magnification. The pecu- 

 liar nature of the posterior corner of the sixth somite also 

 is shown. 



Each cleft lies between two visceral arches. The first or 

 mandibular arch contains a prolongation of the second or 

 mandibular somite. Therefore, since clefts and somites corre- 

 spond, the next posterior visceral arch (hyoid or second) must 

 correspond to the next somite (hyoid or third somite). This is 

 corroborated by the fact that these two consecutive arches, 

 first and second, are related to two consecutive dorsal nerves, 

 trigeminal and facial. Similarly the third visceral arch corre- 

 sponds to the fourth somite and the glossopharyngeal nerve. 

 This interpretation implies that the dorsal roots are related to 



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