212 EDWARD PHELPS ALLIS, JUN. 



tion of the future profundus nerve. The other process grows 

 anteriorly and forms the distal portion of the same nerve. 

 The descending line of neural crest cells is said to be Piatt's 

 thalann'cus nerve^ and it is said to later entirely disappear. 

 The processes that form the profundus nerve do not begin to 

 develop until the descending line of cells that represents the 

 thalamicus has reached the dorsal surface of the optic vesicle^ 

 the profundus nerve thus being first laid down in its adult 

 position. The oculomotorius and trochlearis are later de- 

 veloped as fibrillar processes from neuromasts in the medulla, 

 and take at once their adult relations to the profundus. The 

 nerve that Neal identifies as the ramus ophthalmicus super- 

 ficialis trigemini develops later than the profundus, and is 

 first described in 17mm. embryos, where it "appears as a 

 fibrillar nerve with peripheral nuclei extending from the Gas- 

 serian ganglion just dorsal to the point of exit of the fibres of 

 the r. ophth. profundus V, and passing anteriorly close to the 

 ectoderm below the r. ophthalmicus superficialis VII " (p. 

 233) . The two trigeminal ophthalmic nerves are thus first laid 

 down in their adult positions, and there is no cpestion what- 

 ever of the profundus cutting through the oculomotorius or 

 trochlearis, or of its cutting through the as yet undeveloped 

 eye muscles. The late development of the ophthalmicus 

 superficialis corresponds with Dixon's account of the late 

 development of the frontal branch of the ophthalmic nerve in 

 man, and strongly suggests their being homologous nerves. 

 The ramus ophthalmicus superficialis facialis is said by Neal to 

 develop " in close connection with the skin along what in 

 the head corresponds with the dorso-lateral line of the trunk." 



There is thus nothing in the development of the ophthalmic 

 nerves, so far as known, to definitely indicate either that the 

 portio trigemini of the ophthalmicus superficialis of selachians 

 ever descends, in other verebrates, to the position of a pro- 

 fundus, or that the latter nerve ever ascends to, or simply 

 remains in, the position of a superficialis. 



Leaving aside, now, all question of the possibility of a 

 simple juxtaposition and subsequent fusion of the profundus 



