584 ALICE JOHNSON AND LILIAN SHELDON. 
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. 
12. 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.—Onur 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. vp, oc. 2. We have 
therefore represented them throughout as distinct. 
