DEVELOPMENT OF CRANIAL NERVES IN THE CHICK. 35 



SO that at this stage the nerve is attached to the brain, not by 

 its apex, but by its side, and the nerve presents a conical pro- 

 jection sticking up beyond its point of attachment. 



This conical process gradually shrinks, and by sixty-seven 

 hours is no longer recognisable (fig. 14, VII). Owing to the 

 roof of the brain growing more rapidly than its sides and floor 

 the nerves appear to be gradually driven down towards the base 

 of the brain. This further change is due entirely to relative 

 rate of growth ; there is no further shifting of the nerve attach- 

 ment. Fig. 16 shows the relation of the parts at ninety-three 

 hours ; the rapid growth of the roof of the brain is evidenced 

 by its extreme thinness. 



The above description applies also, as already stated, to the 

 other cranial nerves and to the posterior spinal roots. In my 

 former paper I was unable ''to determine with certainty^' the 

 mode in which the shifting of the roots occurred. Having now 

 traced all the stages, I am perfectly satisfied that the original 

 attachment is lost and a new one acquired. This, I may notice, 

 can hardly be a primitive state of things. 



This shifting of the point of attachment has been noticed also 

 by Balfour in Elasmobranchs. He, however, does not describe 

 a secondary attachment, but considers the shifting as due' entirely 

 to growth of the cells in the median dorsal line of the neural 

 canal. Consequently he finds the stage described above at fifty- 

 four hours, where the nerve is attached by its side to the brain, 

 and projects up above the point of attachment, very puzzling. 

 The occurrence of this stage in Elasmobranchs seems to me to 

 prove that a secondary attachment is acquired in Elasmobranchs 

 in the same manner as I have just described in the chick. 



In figs. 21 and 23 the relations of the facial and auditory 

 nerves are well seen. The facial (VII) is derived from the 

 anterior part of the common root ; it passes downwards and 

 forwards, swelling out into a ganglion just behind the upper 

 end of the hyo-mandibular cleft. Erom this ganglion two 

 branches arise — a larger posterior one, which arches somevphat 

 backwards, and then passes downwards in the hyoidean arch, 

 and a smaller anterior one, which arches over the top of the 

 cleft and then runs down along its anterior wall. 



The part of the combined root which becomes the auditory 

 nerve (VIII) is considerably larger than that from which the 

 facial is derived. It passes downwards and backwards as a large 

 ganglionic mass closely applied to the anterior wall of the 

 auditory vesicle (figs. 26, 23, and 21). 



It is worthy of notice that the nerve comes in contact with 

 the auditory epithelium at a very early period — about fifty hours ; 

 ' Loc. cit., p. 425. 



