EARLY DEVELOPMENT OF RANA TEMPORARIA. 135 
The position of this “ Zwichenstrang” varies in different 
parts of the body and in the head where it lies between the 
neural canal and the epiblast; after uniting with the central 
nervous system! it gives origin to the Gasserian, auditory, 
glosso-pharyngeal, and vagus ganglia. 
This origin hence differs essentially from that given above 
in Elasmobranchs, where the nerve first grows out of the brain 
(i.e. from the neural ridge), and only subsequently fuses with 
the epiblast where the ganglia are formed, and, moreover, this 
fusion takes place at a very different level from that described 
by His as occurring in the chick, and shown by him? also in 
in the section which he figures of Scyllium. He also draws 
two sections of frog embryos, and marks on them the position 
of the “ Zwichenrinne,” but his figures do not show the very 
clear double nature of the epiblast, and it is to this that is due 
the method of development of the peripheral nervous system 
above described. 
It is possible that the condition which obtains in the 
Amphibia* may be an exceedingly primitive one. ° The pos- 
session of what must be regarded as a complete nervous sheath 
points in this direction, and the development in such forms as 
the chick and Elasmobranch, where the epiblast does not sepa- 
rate into its two constituent parts, may be regarded as a con- 
siderably modified one. It is important to notice also in this 
connection that, where in Elasmobranchs the nervous layer of 
the epidermis may be regarded as most developed—that is about 
the level of the lateral line—the cranial nerves here fuse with 
the epiblast, and at the point of fusion the ganglia are 
produced. 
Possibly the neural ridge which grows out from the central 
nervous system may be taken as representing, in a highly- 
modified form, the nervous layer which in Amphibians sur- 
1 Op. cit., figs. iil, a—* 
2 Op. cit., fig. xi, 2. 
3 It is exceedingly likely that in Teleostei and Lepidosteus where the epi- 
blast is also double, that the peripheral nervous system will be found to 
originate in the same way. 
