282 THE ORIGIN OF VERTEBRATES 
and the other connected with the surface-cells, which he calls the 
lateral ganglia. This second set corresponds to Kupffer’s epibranchial 
ganglia. Now it is clear that in the case of the vagus nerve, where, 
as is well shown in Ammoccetes, the nerve is not a single segmental 
nerve, but is in reality made up of a number of nerves going to 
separate branchial segments, the indication of such segments is not 
given by the main vagus ganglion or neural ganglion, but by the series 
of lateral ganglia. So also it is argued in the case of the trigeminal, 
that if in addition to the ganglion-cells arising from the neural crest 
separate ganglion-masses are found in the course of development, 
in connection with proliferating patches of the surface (plakodes, 
Kupffer calls them), then such isolated lateral ganglia are indications 
of separate segments, just as in the case of the vagus, even though 
the separate segments do not show themselves in the adult. So far 
the argument appears to me just, but the further conclusion that the 
presence of such plakodes shows the previous existence of branchial 
sense-organs, and, therefore, that such ganglia are epibranchial 
ganglia, indicating the position of a lost gill-slit, is not justified by 
the premises. If, as I suppose, the trigeminal nerve supplied a series 
of non-branchial appendages serially homologous with the branchial 
appendages supplied by the vagus, then it is highly probable that the 
trigeminal should behave with respect to its sensory ganglia similarly 
to the vagus nerve, without having anything to do with branchie. 
Such plakodal ganglia, then, may give valuable indication of non- 
branchial segments as well as of branchial segments. The researches 
of Kupffer on the formation of the trigeminal ganglia in Ammoccetes 
are the chief attempt to find out from the side of the sensory ganglia 
the number of segments originally belonging to the trigeminal. The 
nature and result of these researches is described in my previous 
paper (Journal of Anatomy and Physiology, vol. xxxiv.), and it will 
suffice here to state that he himself concludes that the trigeminal 
originally supplied five at least, probably six, segments. As I have 
stated there, the evidence as piven by him seems to me to indicate 
even as Many as seven segments. 
In the full-grown Ammoccetes, as is well known, there are two 
distinct ganglia belonging to the trigeminal, the one the ganglion of 
the ramus ophthalnicus, the other the main ganglion. 
According to Kupffer the larval Ammoccetes possesses three sets 
of ganglia, not two, for between the foremost and hindmost ganglion 
