NERVOUS SYSTEM OF THE VERTEBRATA. 



455 



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

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

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

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

 posterior 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 develop- 

 ment of 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 Elasmobranchii, 

 where, moreover, they present a very primitive arrangement. 



hi, 



fy 



FIG. 270. 



TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE 



HEAD OF AN EMBRYO CHICK OF THIRTY HOURS. 



hb. hind-brain; vg. vagus nerve; cp. epiblast; ch. notochord; x. thickening of 

 hypoblast (possibly a rudiment of the subnotochordal rod) ; al. throat ; ht. heart ; 

 //. body cavity ; so. somatic mesoblast ; sf. splanchnic mesoblast ; hy. hypoblast. 



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

 development of the eye. 



