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



be somewhat less marked than in the anterior swelling, but in which the original 

 significance as parts of the central system should still be indicated either by histological 

 or by embryological features. 



To these latter conditions nothing can answer in the Vertebrate nervous system 

 excepting the so-called ramus lateralis vagi. It is present in all Vertebrates above 

 Amphioxus, long and important in the aquatic Ichthyopsida, gradually disappearing 

 when the aquatic medium is exchanged for an air-breathing existence, and finally only 

 retained in the higher Vertebrates as the inconspicuous ramus auricularis vagi. 



Its course is indeed strictly lateral, and has always been a puzzle to anatomists. 

 Stannius^ characterises the existence and the course of this sensory nerve along the 

 trunk down to the tail as " one of the most interesting facts of anatomy." 



None the less startling is its development. Whilst Balfour attempted in this respect 

 to bring it on one line with the other parts of the peripheral nervous system, the 

 corresponding results of Semper, Goette, van Wijhe, and Hofi'mann are all in the 

 contrary direction. They have seen the nervus lateralis appear as an independent 

 product of the epiblast, arising in loco along its whole length, its formation often even 

 preceding that of the spinal nerves. These results have again been fully confirmed and 

 definitely established by the latest investigator of the problem, Beard,^ who also gives a 

 detailed description and figures of the connection between the nervus lateralis and the 

 vagus ganglion, both of them so much more massive and conspicuous in early embryonic 

 stages than later on. 



And now that we are attempting to find out whether there is a possibility of com- 

 paring the lateral nerve-stems of lower worms with the nervus lateralis of Vertebrata, 

 we are naturally led to consider, in the second place, the question whether the anterior 

 swellings of these lateral stems (the paired brain-lobes of the worm) may have their 

 morphological equivalents, their remnants, in the set of anterior nervous swellings that 

 are found in the head on a level with the nervus lateralis, and longitudinally connected 

 with it ; viz. , the variable set of ganglia of the cephalic nerves. 



As to the origin of these ganglia of the cranial nerves I have no observations of my 

 own, and must rely on the data of other observers. 



It is suggestive to give the opinion of the three latest investigators of the develop- 

 ment of these organs in different groups of Vertebrates in their own words. 



Professor A. Froriep,^ who studied Mammalian embryos, writes {loc. cit., p. 35) : — 

 " The ganglia (of facialis, glossopharyngeus, and vagus) enter into a peculiar, very inti- 

 mate connection with the epiderm "; further (p. 40), '• these ganglionic connections with 

 the epiderm must probably be regarded as rudiments of organs which have phylogeneticaUy 



1 Das peripherisehe Nervensystem der Fiache, p. 108. 



' The System of Branchial Sense-Organs, &c., in Ichthyopsida, Quart. Journ. Micr. Sci, November 1885, p. 95. 



' Ueber Anlagen von Sinneeorganen am Facialis, &c., Archivf. Aimt. u. Phys., 1885, Anat. Abth. 



