HEAD 569 



a continuous series with those of the body. The first is prcoral, 

 and the others correspond in position to the mandibular, hyoid, 

 and five branchial arches. The lateral plate mesoblast contains 

 a series of cavities which pass down these arches, which are formed 

 when the gill slits pierce the body-wall intersegmentall\-. Tlic 

 spiracle is a non-functional gill sht anterior to the first, and the 

 mouth appears to be formed by the confluence of two gill slits 

 anterior to the spiracle. Part of the sclerotome grows down and 

 helps to form the appropriate cartilaginous arch, though much 

 of this comes from unsegmented mesenchyme derived from 

 ectodermal cells of the neural crest. The connection of the 

 sclerotomes with the dorsal part of the skull is not clear. The 

 m^^otomes form the muscles of the eyeball, and each of these 

 is supplied by the appropriate ventral root. The dorsal root 

 passes behind the myotome and supplies the muscles of the jaw 

 or branchial arches w^hich have been formed from the lateral 

 plate, and there is usually also a small pretrematic branch running 

 to the segment in front. The dorsal and ventral roots of the cranial 

 nerves thus remain separate throughout life, as the spinal roots 

 do in Branchiostoma and Petromyzon ; in gnathostomes the spinal 

 dorsal and ventral roots fuse. The segmental arrangement of 

 the cranial nerves has no relation to their numbering as it is 

 usually learnt by the student ; moreover, the optic and olfactory 

 nerves are not strictly nerves at all, but extensions of the wall 

 of the brain, and as such have no place in the scheme. The auditory 

 nerve appears to be part of the facial which is specially developed 

 in connection with the acustico-lateralis system of sense organs, of 

 which the ear forms a part ; in the extinct cephalaspids, in the 

 skulls of which the courses of the nerves can be traced, there was a 

 complete series of segmental lateralis nerves. The spinal accessory 

 of mammals is the last branch of the compound vagus in which 

 it is elsewhere included. The blood vessels of the head correspond 

 to the gills and are therefore segmental in position, but in tlie 

 anterior region, where the gills are lost, there is some confusion. 

 The smooth arrangement of the head segments is disturbed in 

 two ways. The preoral, mandibular and hyoid segments are 

 disturbed ventrally by the development of the mouth and loss 

 of the gills which may be presumed to have once been present, 

 and the sense organs upset the dorsal part. The olfactory organ, 

 being terminal, has Httle effect, but the eyes and ears are large 

 and lateral, and leave Httle room for anything else in the segments 



M.Z. 19 



