MORPHOLOGY OF CRANIAL MUSCLES IX SOME VERTEBRATES. 173 



It might be expected that if the lateral plates are the 

 splanchnic elements of myotomes above, their relation to the 

 latter would be constant. But this is not so ; for instance, 

 in the figure given by Ziegler of a 6"5 mm. Torpedo embryo, 

 two laterrd plates (those of the second and third branchial 

 arches) lie beneath three mj^otomes — his fifth, sixth, and 

 seventh (= sixth, seventh, and eighth of van Wijhe), and the 

 lateral plate of the fourth branchial arch lies below his 

 eighth. In Ceratodus, according to Greil, the lateral plates 

 of the first three branchial segments are continuous above 

 with one (the first) myotome, and those of the fourth and 

 fifth branchial segments are continuous above with one 

 myotome (the second). In Ran a esculenta, with five to 

 six somites (Corning, '99, Taf. ix, figs. 7 and 11), the lateral 

 plates of the first and second branchial segments lie in front 

 of the first myotome. To this may be added that in rabbit 

 embryos 3 mm. long (Text-fig. 78) the first branchial lateral 

 plate lies in front of the first trunk myotome, and the second 

 and third branchial lateral plates (as yet not separated) lie 

 beneath the first trunk myotome. 



Secondly, it might be expected that the lateral plate would 

 in all animals be at first continuous with a myotome above, 

 but in Xecturus embryos (Text-figs. 51-53) and rabbit 

 embryos (Text-fig. 78) it is not possible to see any continuity. 



Both these points have been emphasised by Agar in a 

 recent criticism of the theory of van Wijhe. Agar's theory is 

 that the fourth somite of van Wijhe represents the condensed 

 somatic portion of the hinder palingenetic head somites, and 

 he points out that some observers have described more than 

 one somite in this situation, e.g. Brauss, two in Spinax, and 

 Miss Piatt, three iu Acanthias. The difficulty in accepting 

 this theory is that, as above stated, this fourth somite is, in 

 Scyllium, continuous with the lateral plate of the hj^oid 

 segment. 



Ziegler throws doubt on the existence of van Wijhe's fifth 

 myotome — the one which lies above the first branchial seg- 

 ment — on the ground that in Torpedo its cavity is no more 



