THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 135 



2. The olfactory nerve resembles the segmental nerves in undergoing 

 during the earlier stages of its development a very considerable 

 displacement of its root of attachment to the brain, and as this 

 feature is one of the most remarkable characters of these segmental 

 nerves, and is, so far as we know, confined to them, its occurrence in 

 the olfactory nerve must be admitted to be of much weight. 



In both the dogfish and chick the olfactory nerves are clearly 

 recognisable before the cerebral hemispheres have commenced to 

 develop, the nerves at this stage arising from the dorsal part of the 

 sides of the original fore-brain or anterior cerebral vesicle. The 

 hemispheres in the chick are lateral outgrowths of the fore-brain, and 

 are from the first situated above, i.e. on the dorsal side of, the roots 

 of the olfactory nerves ; they grow forwards and upwards with great 

 rapidity, driving the olfactory nerves down to the base of the brain, 

 and so causing these nerves to appear to arise from their under and 

 anterior part. Whether the root of the olfactory nerve undergoes 

 any change comparable to the secondary attachment described above 

 as occurring in the spinal nerves, has, however, not yet been ascer- 

 tained. 



3. The general course of the olfactory nerve in the early stages of 

 development is, like the segmental nerves, at right angles to the axis 

 of the head at the point of origin of the nerve, although, owing to 

 cranial flexure, it is very far from being parallel to the hinder 

 segmental nerves. This feature is shown in fig. 17, I. In the later 

 stages of development, owing to the forward growth of the nasal 

 region this relation becomes completely lost. 



4. Concerning the relations of the olfactory nerve to visceral arches 

 and clefts, I must beg to refer the reader to the paper quoted above, 

 in which I have drawn attention to " the very close resemblance as to 

 form, structure, general relations, time of appearance, &c, existing 

 between the olfactory organ and the gill clefts," and have adduced 

 other arguments on which I have attempted to establish the following- 

 conclusions : — " That the olfactory organ is the most anterior visceral 

 cleft ; that the olfactory nerve is the segmental nerve supplying the 

 two sides of that cleft in a manner precisely similar to that in which 

 the hinder clefts are supplied by their respective nerves ; and that the 

 Schneiderian folds are homologues of gills." 1 



1 " Morphology of Vertebrate Olfactory Organ," Quart. Journ, of Micros. Science, July 

 1879, p. 330. 



