176 JOHN BEARD. 



is the so-called dorsal branch, and, as previously stated, will be here 

 called the siqora-branchial branch. 



10. The sensory thickening of a segment, which gives rise to the 

 branchial sense organs of that segment, may remain very small or may 

 increase to a very considerable length, but in any case the nerve con- 

 necting the whole length of the thickening with its ganglion is split 

 off from the thickening, and split off simultaneously with the growth 

 of the latter. 



1 1 . The pree-branchial nerve is also formed as the ganglion separates 

 from the skin, and is probably in all cases also split off from the 

 epiblast in front of each cleft. 



12. Of the development of the B. pharyngeus nothing can be here 

 recorded ; but I think, from the nature of the case, that this nerve also 

 probably arises from the cells on the upper wall of the cleft. 



Thus, as the general result of these observations, the existing views 

 of the development of the dorsal root of a cranial nerve will have to 

 undergo some modification. That in Elasmobranchs the main root of 

 the nerve is a direct outgrowth from the neural ridge, as stated by 

 Balfour and Marshall, is certainly true. The shifting and acquisition 

 of a secondary point of attachment described by Marshall also seem to 

 take place. The post-branchial branch also appears to arise from the 

 direct outgrowth from the neural ridge, but in the formation of the 

 rest the epiblast probably plays a part. In the case of the supra- 

 branchial branches this is certain, and it is highly probable in the 

 case of the ganglion. That the other branches, viz. the prse-branchial 

 and R. pharyngeus of Van Wijhe, are derived from the skin is pro- 

 bable, and in one case it can be proved, viz. the prse-branchial nerve 

 of the hyoid. 



Having now a general view of the development of a typical cranial 

 nerve, the various nerves may be considered. In the above schema 

 we have the key to all the cranial nerves. Some, such as the ninth, 

 or glossopharyngeal, we shall find to fit in pretty exactly with the 

 schema. But in others the story that ontogeny often omits or distorts 

 ancestral history is also repeated. 



Some of the branches may be absent even in the ontogeny, while 

 others may be abnormally developed. Others, again, may be partially 

 fused with neighbouring nerves, as has been abundantly demonstrated 

 by previous writers. But whatever the adult condition of any of the 

 dorsal roots of the cranial nerves, whatever the actual condition of 



