475 



The eleven anterior neuromeres as above defined have been aniply 

 demonstrated in Acanthias by Locy and by v. Kupffer. These are 

 well marked in the embryo which has been reconstructed in the dia- 

 gram reproduced herewith. The nerves are related to them exactly 

 as they are in the gull, and as they are in fact in so many forms. 

 The sixth nerve is not so well developed to actually determine whether 

 it is really the nerve of the external rectus but its position and its v 

 origin by a series of ventral rootlets associated with the fifth and sixth 

 rhombomeres render its identification certain. There can be no doubt 

 therefore that the ventral root described by Ziegler ^) and Brohmer ^) 

 just below the auditory vesicle in Chlamydoselachus is the abducent. 



As in the gull the first post-otic rhombomere is the seventh, and 

 it gives attachment to the glossopharyngeal. The vagus, after its 

 main connexion with the eighth rhombomere, is joined a second time 

 to the neural crest in the neighbourhood of the ninth rhombomere. 

 The neural crest which joins the IX and X nerves is continued back- 

 wards; it is swollen successively opposite the somites into a series of 

 three rudimentary dorsal roots and then gives off the first distinct 

 ganglion. The pronephric swelling extends forwards to the somite of 

 the third ganglion, and its position was emphasised by the origin of 

 the vitelline artery on the right side from which the reconstruction 

 was made. It is clear, therefore, as has been shown by Braus and 

 by Froriep, that the first dorsal ganglion marks the hinder limits 

 of the head. Acanthias thus presents the same number of rhombo- 

 meres as the gull — 13. 



I venture to think that the results confirm what I previously 

 stated, viz. that the lateral line system has been separated in an 

 anterior direction from the neural crest as far as the ninth usually 

 the eighth rhombomere. Posteriorly to this the neural crest only 

 yields dorsal roots of the spinal series. Anteriorly both the "head 

 dorsal" and the "spinal dorsal" may be present, the latter in a rudi- 

 mentary condition. 



I do not propose in the present paper to enter further into 

 theoretical considerations. But I should like to say that the diagram 

 reproduced here shows that the auditory organ has had evidently a 

 profound influence upon the position of the nerves, the eighth and the 

 seventh being pushed forward as I indicated in the paper on the gull. 



Armstrong College, Newcastle-upon-Tyne, 19th April 1909. 



1) Anat. Anz., Bd. 33, 1908, p. 561. 



2) Jenaische Zeitschr., Bd. 44, 1909. 



