DEVELOPMENT OF ELASMOBRANCH FISHES. 145 



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 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 xii. 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, esta- 

 blished 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 (PI. xii. 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 Avould 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 peripheral manner from the integument. 



This is an important point to make certain of in settKng 

 any doubtful features in the nervous supply of the lateral 

 line. Professor Semper^ with whom as dealing with Elasmo- 

 branchs 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 to the ectoderm, and at the hind end of the 

 body is not yet completely segmented off (abgegliedert) from 

 the ectoderm." Although the last sentence of this quotation 

 ma} seem to be opposed to my statements, yet it appears to mo 

 1 Loc. cit. p. 398. 



