374 CRANIAL NERVED. 



So far as Las been made out, the development of the spinal 

 nerves of other Vertebrates agrees in the main with that in Elasmo- 

 branchii, but no dorsal commissure lias yet been discovered, except in 

 the case of the first two or three spinal nerves of the Chick. 



In the Chick (Marshall, No. 353) the posterior roots, during their early 

 stages, closely resemble those in Elasmobranchii, though their relatively 

 smaller size makes them difficult to observe. They at first extend more or 

 less horizontally outwards above the muscle-plates (as a few of the nerves 

 also do to some extent in Elasmobrancliii), but subsequently lie close to the 

 sides of the neural canal. They are shewn in this position in fig. 116 sp.y. 

 There does not appear to be a continuous crest connecting the roots of the 

 po.sterior nerves. The later stages of the development are precisely like 

 those in Elasmobranchii. 



The anterior roots have not been so satisfactorily investigated as the 

 posterior, but they grow out, possibly by several roots for each nerve, 

 from the ventral corners of the spinal cord, and .subsequently become 

 attached to the posterior nerves. 



I have observed the development of the posterior roots in Lepidosteus, 

 in which they appear as projections from the dorsal angles of the spinal 

 cord, extending laterally outwards and, at first, having their extremities 

 placed dorsally to the muscle-plates. 



The cranial nerves 1 . The earliest stages in the development uf 

 the cranial nerves have been most satisfactorily studied, especially by 

 Marshall (No. 354), in the Chick, while the later stages have been 

 more fully worked out in Elasmobrancliii, where, moreover, they 

 present a very primitive arrangement. 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, connectiog the dorsal end of the incom- 

 pletely 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. 



1 The optic nerves are for obvious reasons dealt with in connection with the deve- 

 lopment of the eye. 



