210 EDWARD PHELPS ALLTS, JUN. 



selachians, the nasal branch being quite unquestionably consi- 

 dered as the homolog'ue of the ophthahnicus profundus, though 

 I do not find this statement definitely made. With equal, or even 

 greater reason, the frontal branch of the nerve in man might 

 be considered as the homologue of the portio ophthalmici pro- 

 fundi of ganoids, and hence more probably the homologue of 

 branch I of the ramus ophthalmicus profundus of Ewart's 

 descriptions of Lsemargus (17) than of the ramus ophthalmicus 

 superficialis trigemini of that fish. Under either assumption, 

 Dixon's facts and conclusions are opposed to the assumption that 

 the superficialis and profundus trigemini of selachians are both 

 represented in the nasalis, or naso-ciliaris, of higher animals. 

 Piatt (49) says that, in Necturus, the ramus ophthalmicus 

 profundus is split off, as a line of cells, from the under surface 

 of the same line of thickened epidermis that later gives similar 

 origin to the ophthalmicus superficialis facialis. In my work 

 on Amia I assumed (3, p. 635), in referring to an earlier 

 work by Piatt (48) , that the two nerves thus described by her 

 were the portio minor and portio major of the ophthalmicus 

 superficialis, and that they retained, throughout life, the posi- 

 tions of those nerves. Piatt's later work shows conclusively 

 that the profundus of her descriptions is the ramus nasalis of 

 Schwalbe's descriptions of other Urodela, in which animals 

 it is said by the latter author to have exactly the relations, to 

 other orbital structures, of the ramus ophthalmicus profundus 

 of selachians. We thus have, under a theory of the develop- 

 ment of nerves opposed to that of His, a nerve that can be 

 followed, if Piatt is correct, from the position of an ophthal- 

 micus superficialis to that of an ophthalmicus pi'ofundus. This 

 profundus nerve is said to differ from all the other sensory 

 nerves of the animal in that it " is formed from the ectoderm 

 in the same manner as are the cranial ganglia" (p. 488), and 

 that neural crest cells participate in its formation throughout 

 its entire length (p. 535). This, it will be seen below, would 

 seem to exclude the possibility of the nerve containing any of 

 the elements of the portio minor of selachians, a descent of the 

 latter nerve thus not here taking place. 



