366 DEVELOPMENT OF ELASMOBRANCH FISHES. 



rous openings to the exterior, or isolated tubes with terminal 

 ampulliform dilatations. 



I have not definitely determined whether the canal-system 

 of the head arises in connection with the lateral line, or only 

 eventually becomes so connected. The important point to be 

 noticed is, that at first no nervous structures are to be seen in 

 connection with it. In stage O nerves for the 'mucous canals 

 make their appearance as delicate branches of the main stems. 

 These nerve-stems are very much ramified, and their branches 

 have, in a large number of instances, an obvious tendency 

 towards a particular sense-organ (PL 13, figs. 5 and 6). 



I have not during stage O been able to detect a case of direct 

 continuity between the two. This is, however, established in the 

 succeeding stage P, in the case of the canals, and the facility 

 with which it may be observed would probably render the 

 embryo Elasmobranch a very favourable object for studying the 

 connection between nerves and terminal sense-organs. The 

 nerve (PL 13, fig. 7) dilates somewhat before uniting with the 

 sense-organ, and the protoplasm of the nerve and the sense- 

 organ become completely fused. The basement membrane of 

 the skin is not continuous across their point of junction, and 

 appears to unite with a delicate membrane-like structure, which 

 invests the termination of the nerve. The ampullae would seem 

 to receive their nervous supply somewhat later than the canals, 

 and the terminal swellings of the nerves supplying them are 

 larger than in the case of the canals, and the connection between 

 the ampullae and the nerves not so clear. In the case of the 

 head, there can for Elasmobranchs be hardly a question that the 

 nerves which supply the mucous canals grow centrifugally from 

 the original cranial nerve-stems, and do not originate in a peri- 

 pheral manner from the integument. 



This is an important point to make certain of in settling any 

 doubtful features in the nervous supply of the lateral line. 

 Professor Semper 1 , with whom as dealing with Elasmobranchs 

 we are more directly concerned, makes the following statement: 

 "At the time when at the front end the lateral nerve has already 

 completely separated itself from the ectoderm, and is situated 

 amongst the muscles, it still lies in the middle of the body close 



1 Loc. cit. p. 398. 



