456 CRANIAL NERVES. 



In the Chick certain of the cranial nerves arise before the 

 complete closure of the neural groove. These nerves are formed 

 as paired outgrowths of a continuous band composed of two 

 laminae, connecting the dorsal end of the incompletely closed 

 medullary canal with the external epiblast. This mode of 

 development will best be understood by an examination of fig. 

 270, where the two roots of the vagus nerve (vg) are shewn 

 growing out from the neural band. Shortly after this stage the 

 neural band, becoming separated from the epiblast, constitutes 

 a crest attached to the roof of the brain, while its two laminae 

 become fused. The relation of the cranial nerves to the brain 

 then becomes exactly the same as that of the posterior roots of 

 the spinal nerves to the spinal cord. 



It does not appear possible to decide whether the mode of development 

 of the cranial nerves in the Chick, or that of the posterior roots of the spinal 

 nerves, is the more primitive. The difference in development between the 

 two sets of nerves probably depends upon the relative time of the closure of 

 the neural canal. The neural crest clearly belongs to the brain, from the 

 fact of its remaining connected with the latter when the medullary tube 

 separates from the external epiblast. 



It is not known whether the cranial nerves originate before the closure of 

 the neural canal in other forms besides the Chick. 



The neural crest of the brain is continuous with that of the 

 spinal cord, and on its separation from the central nervous axis 

 forms on each side a commissure, uniting the posterior cranial 

 nerves with the spinal nerves, and continuous with the com- 

 missure connecting together the latter nerves. 



Anteriorly, the neural crest extends as far as the roof of the 

 mid-brain 1 . The pairs of nerves which undoubtedly grow out 

 from it are the third pair (Marshall), the fifth, the seventh and 

 auditory (as a single root), the glossopharyngeal, and the various 

 elements of the vagus (as separate roots in Elasmobranchii, but 

 as a single root in Aves). Marshall holds that the olfactory 



1 Marshall holds that the neural crest extends in front of the region of the optic 

 vesicle. I have been unable completely to satisfy myself of the correctness of this 

 statement. In my specimens the epiblast along the line of infolding of this part of 

 the roof of the brain is much thickened, but what Marshall represents as a pair of out- 

 growths from it like those of a true nerve (No. 354, PI. n. fig. 6) appears to me in my 

 specimens to be part of the external epiblast ; and I believe that they remain connected 

 with the external epiblast on the complete separation of the brain from it. 



