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Misses A. Johnson and L. Sheldon. 



[Jan. 28, 



III. " On the Development of the Cranial Nerves of the Newt.'' 

 By Alice JOHNSON, Demonstrator of Biology, Newnham 

 College, Cambridge, and Lilian Sheldon, Bathurst Student, 

 Newnham College, Cambridge. Communicated by Pro- 

 fessor M. Foster, Sec. R.S. Received January 14, 1886. 



The peripheral nervous system of the Newt does not begin to 

 develop until the medullary canal has become completely separate 

 from the external epiblast. A neural ridge appears on the dorsal 

 surface of the medullary canal, and, at regular intervals, paired lateral 

 prolongations of it grow out and form the spinal and cranial nerves. 

 The former grow down between the medullary canal and muscle- 

 plates, the original outgrowth, in each case, becoming the dorsal root, 

 while the ventral root is formed later. 



In the head, where there are no muscle-plates, the nerves are 

 situated nearer to the surface. The 3rd, the 5th, the common rudi- 

 ment of the 7th and 8th, the 9th, the 10th, and, probably, the olfac- 

 tory nerves, all develop as outgrowths of the neural ridge. We have 

 not traced the development of the 4th and 6th nerves. 



At first the cranial nerves are of necessity attached to the dorsal 

 surface of the brain, since they arise from the neural ridge. The 

 attachment next widens out, extending further down the side of the 

 brain. Later, the dorsal part of the attachment is lost, the ventral 

 part alone remaining as the permanent root. There is no secondaiy 

 attachment entirely distinct from the first, and no part of the nerve 

 is at any time situated dorsal to the root. This description applies 

 specially to the 5th and 7th nerve-roots, but it is probably true for all 

 the cranial nerves in which the shifting of the roots takes place. 



As soon as the cranial nerves become apparent, a series of thicken- 

 ings of the inner layer of the epiblast appears on each side of the 

 head. These thickenings, which are paired, are situated slightly 

 above the level of the notochord, and correspond respectively to the 

 5th, 7th, 9th, and probably also to the 10th nerves. They are destined 

 to give rise to the mucous canals of the head. The olfactory organ 

 and ear have similar relations to the 1st and 8th nerves, and may 

 possibly belong to the same morphological category as these sense- 

 organs ; but this point seems very doubtful. The mucous canals are 

 at first confined to the head, but afterwards are present in the trunk 

 also, and increase greatly in number. 



The nerves are directed outwards and downwards from the dorsal 

 surface of the brain, each towards the sensory epiblastic thickening 

 corresponding to it. After a short time, the two structures fuse 

 together indistinguishably, so that a mass of cells is formed at this 

 point, continuous, on the one hand, with the external epiblast, and, on 



