140 THE VOYAGE OF H.M.S. CHALLENGER. 



skin, the sensory thickening begins to grow backwards along the lateral surface of the 



trunk. This thickening is the rudiment of the so-called lateral line The 



so-called lateral nei-ve is formed from the deeper portion of the sensory thickening. 

 . . . . That there is no actual growth backwards of the nerve is obvious enough." 



Eecapitulating, we must acknowledge that the mode of origin of the ganglia of the 

 cephalic nerves, as described by these authors, is certainly a peculiar one — a mode of 

 development sui generis. One of Beard's accompanying diagrammatic figures, repro- 

 duced in Wiedersheim's second edition (1886) of the Lehrbuch der Vergleichenden 

 Anatomic as woodcut No. 265, moreover, shows how the position of the cephalic ganglion, 

 developing as an ectodermal proliferation, is in this early stage eminently lateral, a 

 conclusion corroborated by the figures of his actual sections. This primitive position is, 

 of course, gradually lost, and could never be predicted from a study of these ganglia and 

 their position and significance in the adult animal. Yet it is not without significance, 

 when seen in the light of the suggestion here brought forward. And that the interpre- 

 tation of the phenomena in question as given by these authors is not universally accepted, 

 thus leaving room for new suggestions, is proved by the following citation from Ransom's 

 and dArcy Thompson's latest article,^ running, as follows: — "Although the lamprey 

 presents a well-marked lateralis nerve, it has not also a regular lateral line, for the sense- 

 organs of the skin are scattered and without segmental arrangement. The sense- 

 organs do not, therefore, appear to be in direct relation with the sjiiual ganglia, and the 

 view of the close connection between them (Spencer, Beard, Froriep) does not receive 



support It seems more natural to consider the lateralis as a relic of the 



extensive and irregular commissure system connecting the posterior roots of Amphioxus." 



Passing from a consideration of the embryonic ganglia to their connection in the 

 adults, I must mention the connection of the ramus lateralis vagi with cephalic nerves 

 anterior to the vagus. I will not here give a description of the numerous varieties pre- 

 sented by this nervous connection, but merely refer to the arrangement in Vertebrates so 

 low as the lampreys. We there find, according to Johannes Miiller, the ramus lateralis 

 originating from the seventh as well as from the tenth pair of cephalic nerves, and if we 

 compare the very satisfactory figure which was only lately" given by Ahlborn of this 

 arrangement, we must recognise that this nervous connection is important, and has more 

 the aspect of a direct forward continuation of the nervus lateralis than of a sensory branch 

 from the facialis, establishing a connection between it and the vagus. 



Ahlborn mentions the existence of a similar connecting stem reaching further forward 

 still, and connecting the trigeminus and facialis. How these connections vary in the 

 different adult Vertebrata will not be discussed here. 



The difi"erent facts and speculations here brought forward in connection with the 



' Ou the Spinal and Visceral Nerves of Cyclostomata, Zool. Anzeiger, No. 227, July 18S6. 

 - Zeitschr. f. wiss. Zool., Bd. xl., pi. xviii. 



