1 88 Journal of Comparative Neurology and Psychology. 



shifting forward of myotomes in the neck region following upon 

 the reduction of postauditory myotomes. 



k. The number of somites in the head can not be stated 

 for vertebrates in general, since the caudal limit of the head 

 varies from group to group. This will be further discussed be- 

 low (Sec. 12). 



J. Branchial apparatus and lateral musculature . 



The position of the gill slits with reference to the m}-o- 

 tomes has been one of the most vexed and vexing questions 

 of head morphology. The most direct and natural assumption 

 is that dorsal and lateral mesoderm had primitively a common 

 segmentation and that the gill slits were situated between the 

 mesodermic segments. Each branchial arch would thus be 

 continuous dorsally with a myotome. This condition, if it ever 

 existed throughout the branchial region, is now preserved only 

 in the case of the mandibular arch and somite 2, and in some 

 forms in the case of somite 3 and the hyoid arch. Caudally 

 to this the branchial arches bear no definite relation to the so- 

 mites, in most vertebrates. This is certainly due in part at 

 least to the reduction and shitting of postauditory myotomes 

 mentioned above and to the growth and expansion of the bran- 

 chial apparatus itself. These two processes result in a shifting 

 forward of myotomes and backward of gill pouches and arches, 

 not only in relation to one another but also in relation to other 

 organs (58, 60, 12). Since such shifting is observed in the 

 ontogeny it may be expected that the primitive condition would 

 be found in very young embryos. In most vertebrates, how- 

 ever, such a direct correspondence between somites and bran- 

 chial arches as was supposed above does not exist at the time 

 when the structures in question are formed. These facts have 

 led some authors to the conclusion that the branchial segmenta- 

 tion is independent of that of the dorsal mesoderm. 



The writer is unable to harmonize this conclusion with the 

 arrangement of the cranial nerves. Also, this conception im- 

 plies a distinct and divergent dorsal and ventral segmentation 

 for the typical vertebrate : the mesodermic segmentation proper 



