262 ALICE JOHNSON AND LILIAN SHELDON. 



5. The development of the peripheral nervous system is 

 preceded by the appearance of a neural ridge, extending along 

 the whole length of the body. . 



6. The spinal nerves grow out from the neural ridge, and 

 pass downwards between the neural canal and muscle plates. 



7. The cranial nerves also grow out from the neural ridge, 

 but are nearer to the surface than the spinal nerves, owing to 

 the absence of muscle plates in the head. 



8. When each has attained a certain length it fuses with a 

 thickening of the epiblast, situated some distance above the 

 level of the notochord. (This is the case with the 5th, 7th, 

 and 9th nerves, and probably also with the vagus.) 



9. At the point of fusion there is a thickening of the nerve- 

 trunk, forming a ganglion, which afterwards recedes from the 

 surface, remaining, however, attached to the sense organ by a 

 nerve. 



10. The main trunk of the nerve passes on, and, in the cases 

 of the 7th and 9th nerves, fuses again with the epiblast of the 

 dorsal wall of the corresponding gill-cleft. Later, the nerve 

 becomes detached from the epiblast, and gives off two branches, 

 one behind and one in front of the gill-cleft. 



11. The 5th nerve has no such second (ventral) fusion with 

 the epiblast, but divides below its first (dorsal) fusion into two 

 branches, the superior and inferior maxillary. 



13. In the Frog a neural ridge is present at an early stage, 

 just after the closure of the neural canal. The facio- auditory 

 nerve grows out of the brain, and it is therefore probable that 

 the other cranial nerves have the same origin. 



N.B. — Our figures are diagrammatic in so far that the outlines 

 of the cells were not perfectly apparent in all sections. This 

 appeared to us to be due to bad preservation, as the better the 

 specimens were preserved the more distinct and complete were 

 the cell outlines. It was generally possible to draw them accu- 

 rately with a camera and Zeiss obj. d, oc. 2. We have 

 therefore represented them throughout as distinct. 



