DEVELOPMENT OF THE NERVOUS SYSTEM. 65 



Somite 8 or 9 and several following it send muscle buds ventrad 

 and forward beneath the branchial apparatus to form the sub- 

 branchial muscles as in cyclostomes. In higher vertebrates 

 the abortion of certain postauditory somites is more complete 

 and the number which disappear may be greater. In various 

 classes of vertebrates more or fewer vertebral segments may be 

 secondarily fused with the primordial cranium, entering into the 

 occipital region of the skull. In mammals and perhaps man four 

 such vertebrae are added to the cranium. In all vertebrates 

 the sub-branchial or tongue musculature is formed from muscle 

 buds derived from post-branchial somites. 



In vertebrates above cyclostomes the breaking down of somites 

 behind the ear produces a shifting of the relative position of the 

 somites and nervous organs. The auditory sac is known to shift 

 backward during development through the length of from one to 

 three hindbrain neuromeres (Fig. 18). Also the permanent somites 

 shift forward somewhat so that at every stage the existing somites 

 lie close behind the auditory sac. The shifting forward of 

 somites at the same time affects their position relative to the 

 branchial apparatus. Also, the disappearance of somites leads 

 to the disappearance of the ventral motor nerves which should 

 innervate them, and apparently the shortening process in the 

 region behind the ear leads to a decrease of the ectodermal area 

 and the reduction or disappearance of one or more nerves of the 

 skin. Finally, the development of the hypoglossal nerve is deter- 

 mined by the migration into the sub-branchial region of the 

 muscle buds which it is destined to innervate. 



The lateral mesoderm in the meantime becomes segmented 

 in a passive manner by the formation of the gill slits. In most 

 vertebrates these slits seem to have no definite position relative 

 to the somites. There is evidence, however, that the segmentation 

 of the lateral mesoderm and pharynx in primitive vertebrates 

 corresponded to that of the dorsal mesoderm. The mandibular 

 and hyoid arches are connected with the second and third somites 

 respectively. The third branchial arch is connected with the 

 fourth somite, and in Petromyzon the total number of gill arches 

 (eight) corresponds to the number of somites (somites 2 to 9) 

 S 



