226 EDWARD PHELPS ALLIS, JUN. 



Petromyzon (20 j. In Bdellostoiim the " Stanun cles Oplithal- 

 micus " is said by Flirbriiiger (p. 31) to run forward over tlio 

 opticus, and his reference to Miiller (a work I have not at my 

 disposal) leads one to suppose that this stem contains motor 

 fibres. This statement, attributed to Miiller, certainly deserves 

 to be controlled, and if found to be correct, the homologies of 

 the muscles so innervated well deserve to be carefully estab- 

 lished. That they can be the homologues of muscles that are 

 innervated in Myxine by a nerve that lies ventral to the 

 opticus I wholly doubt, and yet this is always assumed to 

 be the case. Pollard even cites this particular case in support 

 of his propositions (52, p. 397) that "The topographical posi- 

 tion and course of nerves is not of great importance ; " and 

 that " the fundamental grounds for determining the homology 

 of nerves are, (1) origin from homologous nerve-cells, (2) 

 terminal distribution to definite structures. The course of 

 the fibres is of less importance." And so impressed is this 

 author with this very easy manner of accounting for the 

 apparently anomalous positions of certain nerves, that he 

 does not even attempt to show either that the cells of origin 

 or the structures innervated are homologous. The reasoning 

 is that the structures innervated must be homologous, because 

 otherwise the nerves would not be. Much more rational 

 assumptions would be, either that there is some error in the 

 descriptions of Bdellostoma, or that the muscles innervated 

 are not homologous. As to Myxine, it Avould seem as if 

 certain motor and sensory fibres of the truncus maxillaris 

 trigemini were simply juxtaposited to the ramus ophthalmicus, 

 as that nerve issues from the skull, and that they later take 

 their proper course ventral to the opticus, leaving the true 

 ophthalmic fibres to course above that nerve. Such a juxta- 

 position is shown by Herrick (29) in Amblystoma, and is said 

 to occur in Cryptobranchus also. In these two cases there 

 is certainly no question of anything more than a simple 

 juxtaposition of the two nerves concerned, and I believe 

 that a similar explanation will be found to hold in all other 

 similar cases. Pollard himself even says (p. 400) of this 



