436 DEVELOPMENT OF ELASMOBRANCH FISHES. 



head which have, relatively speaking, undergone but slight 

 modifications, and which require no special elucidation from the 

 nerves, that these sufficiently retain in the adult their primitive 

 form to serve as trustworthy morphological guides. 



I propose to examine separately the light thrown on the 

 segmentation of the head by the development of (i) the nerves, 

 (2) the visceral clefts, (3) the head-cavities ; and then to compare 

 the three sets of results so obtained. 



The post-auditory nerves present no difficulties ; they are all 

 organized in the same fashion, and, as was first pointed out by 

 Gegenbaur, form five separate nerves, each indicating a seg- 

 ment. A comparison of the post-auditory nerves of Scyllium 

 and other typical Elasmobranchs with those of Hexanchus and 

 Heptanchus proves, however, that other segments were originally 

 present behind those now found in the more typical forms. And 

 the presence in Scyllium of numerous (twelve) strands from 

 the brain to form the vagus, as well as the fact that a large 

 section of the commissure connecting the vagus roots with the 

 posterior roots of the spinal nerves is not connected with the 

 brain, appear to me to shew that all traces of the lost nerves 

 have not yet vanished. 



Passing forwards from the post-auditory nerves, we come to 

 the seventh and auditory nerves. The embryological evidence 

 brought forward in this paper is against regarding these nerves 

 as representing two segments. Although it must be granted 

 that my evidence is not conclusive against an independent 

 formation of these two nerves, yet it certainly tells in favour of 

 their originating from a common rudiment, and Marshall's results 

 on the origin of the two nerves in Birds (published in the 

 Journal of Anatomy and Physiology, Vol. XI. Part 3) support, 

 I have reason to believe, the same conclusion. Even were 

 it eventually to be proved that the auditory nerve originated 

 independently of the seventh, the general relations of this 

 nerve, embryological and otherwise, are such that, provisionally 

 at least, it could not be regarded as belonging to the same 

 category as the facial or glossopharyngeal nerves, and it has 

 therefore no place in a discussion on the segmentation of the 

 head. 



The seventh nerve of the embryo (PI. 17, fig. I, vn.) is 



