124 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



— Cameron and Milligan) there is evidence that the same mode of 

 development holds. One of the important points to be settled is 

 whether the nuclei which are seen scattered about in the young 

 nerve-trunk and which give it a syncytial appearance are not really 

 immigrant sheath nuclei. The conditions in Lepidosiren where it is 

 easy to distinguish the heavily yolked sheath protoplasm make it 

 appear probable that this will be found to be the case. 



II. The optic nerve is not a true peripheral nerve comparable 

 with the other cranial nerves but"- simply a narrow isthmus or stalk 

 connecting the main brain with its outlying portion which forms the 

 retina. Its development is mentioned in the description of the eye. 



III, IV and VI. The oculomotor, pathetic and abducent nerves 

 appear to agree exactly in their main developmental features with 

 ordinary motor nerves of the trunk (Neal, 1914). 



It is not proposed to say anything here regarding the topography 

 of the cranial nerves but some points regarding it will be touched 

 upon later on in connexion with the segmentation of the head. 



Sympathetic. — The sympathetic ganglia, as was first shown by 

 Balfour (1878) for Elasmobranchs, are derived directly from the 

 spinal (or cranial) nerves. In its earliest recognizable stage the 

 ganglion forms a swelling on the course of the nerve just ventral to 

 and continuous with the spinal ganglion. With further development 

 the ganglion bulges more and more pronouncedly towards the mesial 

 plane at about the level of the dorsal aorta. The nerve-trunk in 

 this region now splits longitudinally and the ganglion becomes 

 shifted farther towards the mesial plane, lying immediately over the 

 posterior cardinal vein and remaining connected by a slender bridge 

 — the ramus communicans — with the spinal nerve from which it 

 has become split off. 



In Sauropsida the sympathetic ganglia arise in similar fashion. 

 In Amphibia and Sauropsida, where the sympathetic ganglia are in 

 the adult connected by a longitudinal trunk, this latter is said to 

 arise secondarily, the ganglia being at first quite separate. In view, 

 however, of the difficulty of detecting such nerve-trunks in early 

 stages of development it will be well not to dismiss altogether the 

 possibility that the ganglia are after all in continuity from the 

 beginning. 



From the basis of the sympathetic system so laid down extensions 

 apparently sprout out into the various tissues which are eventually 

 innervated by this system — but again there has to be admitted great 

 possibility of error. The problem of the mode of development of 

 these obscure portions of the nervous system will probably only be 

 satisfactorily settled after we know with certainty the processes at 

 work in the development of the main nerve-trunks and ganglia. 



The Okgans of Special Sense.— We may take it that in the 

 early stages of the evolution of the nervous system, while this 

 system was still a diffuse network, there were already present 

 scattered sensory cells' — cells specialized for the reception of im- 



