MOHPHOLOGY OF CRANIAL MUSCLES IN SOME VEETEBEATES. 175 



than a cleft, and that it quickly becomes rudimentary. He 

 concludes that it is only a small cavity in the mesenchyme 

 and of no theoretic importance. It is difficult to share this 

 opinion, for the corresponding structure in Scyllium is lined by 

 epithelial cells and closely resembles the next following 

 myotome (Text-figs. 1 and 2). 



Ziegler counts three myotomes in Torpedo (his fifth, sixth, 

 and seventh) corresponding to three lateral plates, regards 

 the vagus as a '^zusammengesetzte Nervencomplex'^ corre- 

 sponding to three segments, and is of opinion that it is in 

 correspondence Avith this that the next following myotome 

 (his eighth = van Wijhe's ninth) is the foremost to have any 

 nerve-roots, an anterior one only. The difficulties in accept- 

 ing such a view are : first, as stated above, the Avant of antero- 

 lateral correspondence between the myotomes and lateral 

 plates; and secondly, in Scyllium, and probably also in Tor- 

 pedo at a later stage, a fiftli branchial lateral plate is formed, 

 i. e. there would be an overlapping of the territories of the 

 vagus and spinal roots. 



Gi-eil's views as to the nature of the mesoderm of the head 

 of Ceratodus are very different from the foregoing. He holds 

 that the musculature of the branchial i-egion is derived from 

 downgrowths of the first two myotomes, that in the first 

 three branchial arches being derived from the first myotome, 

 that in the fourth and fifth arches from the second myotome. 

 The cells forming these downgrowths can be distinguished 

 fi'om the in)mediately subjacent lateral plates by the shape of 

 their nuclei and by the later absorption of their yolk-granules. 

 The downgrowths increase in vertical depth from behind 

 forwards. The lateral plates of the branchial arches degene- 

 rate into connective tissue. The dorsal portions of the first 

 three arches, i. e., those which are formed from the first 

 myotome, develop dorsally into the levatores arcum branchi- 

 alum and ventrally into the second and third interbranchi- 

 ales and the ceratohyoideus ; that of the fourth arch into the 

 fourth levator and fourth interbranchialis ; that of the fifth 

 arch anteriorly into tlie dorso-laryngeus and the fifth inter- 



