66 NERVOUS SYSTEM OF VERTEBRATES. 



in front of that one which forms the first sub-branchial muscle 

 bud. The space occupied by the branchial apparatus, however, 

 is always greater than that occupied by the corresponding number 

 of somites and there is apparently a lack of correspondence between 

 muscle segmentation and gill segmentation. The reason for this 

 is found first in the fact that the gills become voluminous structures 

 which require more space than the corresponding number of 

 somites, and second in the disappearance and shifting of somites 

 in the postauditory region mentioned above. 



The segment of the lateral mesoderm between each two . gill 

 clefts is called a branchiomere and gives rise to the skeleton and 

 muscles of the branchial arch. The mouth is regarded as repre- 

 senting a pair of gill slits and the mandibular arch as a branchial 

 arch. The muscles of mastication which are attached to the 

 lower jaw are derived from the lateral mesoderm and these together 

 with most of the muscles of the branchial arches are visceral 

 muscles, homologous with those in the wall of the alimentary 

 canal in other segments of the body. In front of the mouth some 

 rudimentary structures have been found in selachians which 

 are thought to be vestiges of brancial arches which once existed 

 anterior to the mouth. Possibly two such arches were present in 

 primitive vertebrates. 



The shifting of the auditory vesicle mentioned above changes 

 its position with regard to the branchial arches also. From 

 lying over the hyoid arch or second gill slit, it comes to lie over the 

 first branchial arch or third gill slit. The bones of the middle 

 ear in mammals are regarded as modified portions of the skeleton 

 of the mandibular and hyoid arches. Recent researches show, 

 however, that in the rabbit the stapes is a derivative of the 

 auditory capsule. 



It is believed that the ancestors of vertebrates had a mouth 

 farther forward than the present mouth. The ancestral mouth 

 is represented by a vestigeal structure formed in embryos and 

 retained in the adult as a rudimentary organ, the hypophysis. 

 The hypophysis makes its appearance as a median ectodermal 

 pit slightly ventral to the olfactory pits. Opposite this pit there 

 lies in selachians a median mass of entoderm connecting the two 



