HEAD OF SQUALUS 465 



cannot bu any other than the sixth, provided that no great 

 migration on the part of the somites has taken place. Several 

 authors regard the seventh somite as the one which gives rise 

 to the first permanent myotome, and this suggests a migration 

 forwards, as described by Braus (2). But, as stated above, 

 any relative movement which the somites undergo is backwards 

 and not forwards, and is entirely passive. Text-figs. 7 and 8 

 are reconstructions of embryos in the 8 mm. and 10 mm. stages 

 respectively. The sixth somite is very obvious, with its 

 anterior margin drawn out and indistinct, anterior to which 

 there are remnants of muscle-fibres of somite 5, and the same 

 is true of the 20 mm. stage (Text-fig. 9). 



Up to a stage between 8 mm. and 10 mm. a ventral root 

 can be traced to the myotome of the sixth somite. In later 

 stages, however, I have been unable to find it. This is in 

 agreement with Neal (11) and Hoffmann (9), both of whom 

 state that it disappears. Presumably the sixth somite is 

 innervated by a branch from the next posterior ventral root 

 and somite (seventh), or the two nerves may combine, but 

 I have not succeeded in determining this point. 



The fate of the ventral roots of the post-otic myotomes is 

 of importance in determining the posterior limit of the cranium. 

 Cartilage begins to appear in the L stage (Sewertzoff, 17 ; 

 Gaupp, 5). Text-fig. 10 shows a reconstructed embryo about 

 50 mm. long. Three ventral roots are present, emerging through 

 foramina in the cranial cartilage. The anterior one is very 

 thin and belongs to the seventh somite, the remaining two are 

 stout nerves. The foramina through which they pass become 

 confluent with the vagus foramen, the vagus lying immediately 

 lateral of their point of exit from the neural tube. These 

 two nerves I regard as belonging to somites 8 and 9. These 

 results are in agreement with those of Hoffmann (9). Fiir- 

 bringer (4) did not study very young specimens of Squalus, 

 so that it is probable that his y and z are the same as Hoff- 

 mann's c and d and my eighth and ninth. 



The next ventral root, the tenth, comes out of a deep notch 

 in the posterior wall of the cranium, but behind the occipital 



