438 PAUL S. McKIBBEN 



The material used consisted of adult specimens of the smooth 

 dogfish (Mustelus canis) and spiny dogfish 'pups' (Squalus 

 acanthias). Adult specimens of Mustelus canis have been used 

 as the basis for all the figures and descriptions which follow. 



The fish were taken from the water, strapped in a specially 

 constructed trough and clamped at the root of the tail. The 

 tail was then cut off about two centimeters caudal to the cloacal 

 aperture and a solution of methylene blue in salt solution in- 

 jected with a pressure bottle into the caudal artery. A 0.075 

 per cent solution of methylene blue (Griibler's "rect., nach Ehr- 

 lich") in 2 per cent salt solution was found most useful. After 

 a successful injection of the nasal region the mucous surface 

 of the nostrils was found to be colored quite blue. The head of 

 the fish was now cut off, the cranium opened and the nervus 

 terminalis dissected off with the aid of a stereo-binocular micro- 

 scope. The nerves were fixed in cold ammonium molybdate, 

 dehydrated rapidly and mounted in balsam or embedded in 

 paraffine. For further details concerning this method see Wilson 

 ('10) and McKibben ('13). About twenty fish were thus in- 

 jected and the study of thirty whole mounts of separate nerves 

 has furnished the basis for the observations which follow. 



For the examination of the whole mounts, the Zeiss binocular 

 stereoscopic ocular used with high power apochromatic objectives 

 was found exceedingly useful. With this apparatus it was 

 found that the continuity and arrangement of the processes of 

 the ganglion cells could be studied much more easily and accu- 

 rately than with a simple ocular. 



The arrangement of the ganglion cells along the nerve varies 

 with each individual and on the two sides of the same head. 

 In three cases nerves were studied which showed a single ganglion. 

 In the others, although usually one group was present which 

 was larger than the rest, the cells were arranged in several ganglia. 

 The main ganglion, as Locy has indicated, is situated usually 

 near the olfactory bulb. Three, four, five and six well-marked 

 ganglia have been observed on a single nerve and in one case 

 ten relatively isolated groups of cells have been encountered. 



