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 branchiae. 



Such plakodal ganglia, then, may give valuable indication of 11011 - 

 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 given 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 gauglion of 

 the ramus ophthalmicus, 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 



