EARLY DEVELOPMENT OF RANA TEMPORARIA. 129 
nerves can be perceived; there is simply the nervous sheath of 
the epiblast surrounding the whole body and connected with 
the lateral edge of the neural plate, and(as this gradually bends 
up on each side) with the dorsal lips of the neural groove 
(fig. 7), and finally, with the dorsal surface of the neural canal 
(figs. 11 and 14). 
In the chick! before closure of the neural canal a neural 
ridge is present on each side of the brain, and from this as 
outgrowths the nerves are formed; in Elasmobranchs,” though 
developed somewhat later, a well-marked neural ridge is present 
and the origin of the peripheral nervous system apparently 
corresponds closely with that of the chick, but in Amphibia I 
have been unable to discover any such structure as a neural 
ridge, and it is only when the neural folds have met and fused 
completely, and the neural canal quite separated off from the 
epidermic layer that the earliest rudiments of nerve appears. 
This earliest appearance does not assume the form of a 
direct outgrowth from the substance of the neural canal, but 
a series of consecutive transverse sections shows that along 
certain lines the cells of the nervous layer proli- 
ferate, and it is by this proliferation that the rudi- 
ments of the cranial nerves are laid down. 
This is most clearly seen in the case of the trigeminal and 
facial nerve, and figs. 16—20 represent stages in the develop- 
ment of the former nerve. 
In fig. 16, on the left side, the nervous layer of the epi- 
dermis is seen to have proliferated (for, save at the part where 
nerves are formed and along the lateral line, it is only one cell 
thick on the sides of the embryo), and it is seen further 
that this proliferation extends for almost the whole length of 
the side. 
The section is an oblique one so that one side passes through 
the optic vesicle, and here the nervous layer is only one cell 
thick except where at the level of the lateral line the very 
front part giving rise to the Gasserian ganglion is cut through. 
1 Marshall, ‘ Quart. Journ. Mic. Sci.,’ vol. xviii, 1878. 
2 Balfour, ‘Monograph on Elasmobranch Fishes,’ London, 1878. 
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