THE SPINAL AND CRANIAL NERVES. 129 



nerve is perhaps to be added to these. The optic nerve develops 

 in a very special manner ; and the mode of development of the 

 third, fourth, and sixth nerves in the frog has not yet been 

 determined with accuracy. 



The trigeminal, facial, glosso-pharyngeal, and pneumogastric 

 nerves, although arising from the neural ridges in the same way 

 as the dorsal roots of the spinal nerves, yet differ from these, and 

 agree amongst themselves, in certain important features, of 

 which the following are the principal : 



i. The nerves in question, in place of growing downwards, 

 like the spinal nerves, alongside the central nervous system, 

 grow outwards, close to the surface of the embryo, between the 

 epiblast and the mesoblast. 



ii. Each of these four nerves acquires a new connection with 

 the surface epiblast some considerable distance beyond the root 

 of origin from the brain, and at about the horizontal level of the 

 notochord ; at this place, and at any rate in part from the surface 

 epiblast itself, the ganglion of the nerve is formed. 



iii. The nerves have special relations to the gill-slits, each 

 nerve dividing into two main branches, which embrace between 

 them one of the gill- slits. 



iv. A special system of cutaneous nerves is developed from 

 the surface epiblast in connection with these four nerves, form- 

 ing the lateral line system of nerves. 



In dealing with the several cranial nerves individually it will 

 be convenient to consider them in order from behind forwards. 



X. The pneumogastric, vagus, or tenth cranial nerve. This 

 grows rapidly in the early stages, and soon attains an enormous 

 size. In embryos of about 3 mm. length (cf. Figs. 58, C, and 

 59), when the neural folds have not quite met in the hinder part 

 of the head, and the neural groove is, therefore, still open, the 

 pneumogastric nerves are already present as a pair of wing- 

 like expansions of the neural ridges. The root of attachment 

 of the nerve, in the re-entering angle at the top of the brain, 

 between the epiblast and the brain wall, is slender ; but the rest of 

 the nerve is of great thickness. It extends more than half way 

 down the side of the pharynx, lying between the mesoblast and 

 the surface epiblast, very close to the latter but distinct from 

 it along its entire length (c/. Fig. 79, x). The nerves of the 

 two sides are in some cases unequally developed at this stage. 



K 



