164 DEVELOPMENT OF THE FROG'S EGG [Cn. XV 



Marshall ('93) says the cranial nerves, "which are undoubtedly 

 derived from the neural ridges, are the trigeminal, the facial 

 and auditory, and the sensory branches of the glossn-plmi-vii- 

 geal and pneumogastric nerves." These 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." 



" 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, 

 forming the lateral line system of nerves." 



The pneumogastric nerves are "wing-like" expansions of the 

 neural plate, extending more than half-way down the side of 

 the pharynx. At the time when the larva leaves the capsule, 

 a thickening of the ectoderm on each side opposite this nerve 

 and at the level of the notochord develops, and fuses with the 

 nerve. From this double origin arises the ganglion of the 

 pneumogastric. A lateral line thickening has appeared as a 

 solid cord of cells on each side, extending from the pneumo- 

 gastric backward along the side of the embryo. 



It is not possible to enter here into the details of the develop ? 

 ment of the other cranial nerves enumerated above. The 

 development of the first, third, fourth, and sixth nerves has 

 not as yet been fully worked out. The origin of the optic 

 nerve has been described in connection with the development 

 of the eye. 



