DEVELOPMENT OF CRANIAL NERVES TN THE CHICK. 33 



stages. At one hundred, ninety-six, and ninety-three hours it has 

 very much the same appearance and relations as at 123 hours 

 (fig. 25). 1 have not yet detected it in any specimen earlier than 

 ninety-three hours. 



6. A transverse section of the brain passing through its roots 

 passes also through that of the seventh nerve. Such a section 

 shows that the two sixth nerves arise very close to the mid 

 ventral line, and are separated from the roots of the seventh by 

 a considerable interval. 



From a careful investigation of the relations of this nerve I 

 have been led to the conclusion that the sixth nerve bears the 

 same relation to the seventh that the anterior root does to the 

 posterior root of a spinal nerve. And since we know the seventh 

 to correspond to the posterior root of a spinal nerve we may 

 speak of the sixth as the anterior root of the seventh. However, 

 as I have not noticed the early stages of development of this 

 nerve, I do not wish to speak too confidently, especially as my 

 conclusions are in direct opposition to Balfour's definitely ex- 

 pressed opinion,^ that no anterior roots exist to the cranial 

 nerves. My own decided opinion is that the sixth is an anterior 

 root, but in the absence of direct embryological proof I must 

 leave the question open. 



There are great difficulties in the way of the sixth belonging 

 to the seventh nerve, the chief of which is the extension forward 

 of the former to the rectus externus. My sections, however, 

 seem to leave no doubt on this point ; the hindmost of the roots 

 of origin of the sixth is but a very short distance in front of the 

 auditory capsule; however, the foremost of these roots appear 

 to be situated in front of the seventh, so that it is possible 

 that the sixth represents the combined anterior roots of the 

 seventh and fifth, in which case the difficulty just noticed 

 vanishes. 



Lockhart Clarke^ describes and figures the sixth nerve as 

 arising in the adult from the same nucleus as the seventh, men- 

 tioning also that the sixth nerve has a large number of roots, 

 strongly suggesting comparison with an anterior spinal root. 

 Meynert^ has also pointed out and figured the same thing. We 

 are as yet ignorant of the relations between the nerve-nuclei of 

 the adult and the primitive embryonic roots of origin of the 

 nerves, but the above statements manifestly lend support to 

 there being some close connection between the sixth and seventh 



* 'Phil. Trans.,' vol. 166, part i, p. 189, note, and 'Journal of Auat. 

 and Pliys.,' vol. xi, p. 459. 



- 'Piiil. Trans.,' 1S6S. 



^ Strieker's ' Histology,' English translation, vol. ii, fig. 254, p. 493, also 

 Quain's 'Anatomy,' 8th edition, vol. ii, p. 513, fig. 359. 



VOL. XVIII. NEW SER. C 



