X ENVOI'S SYSTEM OF T LIE VKRTEIiRATA. 



The growth of the cerebral hemispheres causes its point of insertion in 

 the brain to be relatively shifted; and on the development of the olfactory 

 lobes (vide p. 366) it arises from them (fig. 273). In Elasmobranchs there 

 is a large development of 

 ganglion cells near its root. 

 From Marshall's figures 

 these appear also to be 



present in the Chick, but 

 they do not seem to have 

 been found in other forms. 

 In both Teleostei and Am- 

 phibia the olfactory nerves 

 are at first extremely short. 

 Marshall holds that the 

 olfactory nerve is a seg- 

 mental nerve equivalent to 

 the third, fifth, seventh etc. 

 nerves. It has been al- 

 .ready stated that in my 

 opinion the origin of the 

 olfactory nerves from the 

 fore-brain, which I hold to 

 be the ganglion of the pras- 

 oral lobe, negatives this 

 view. The mere fact of 

 these nerves originating as 



an outgrowth from the central nervous system is no argument in favour of 

 Marshall's view of their nature; and even if Marshall's opinion that they 

 arise from the neural ci'est should turn out to be well founded, this fact 

 would not prove their segmeiital nature, because their origin, from this 

 crest would, as indicated in the next paragraph, merely seem to imply that 

 they primitively arose from the lateral borders of the nerve-plate from 

 which the cerebro-spiiial tube has been formed. 



Situation of the dorsal roots of the cranial and spinal nerves. 



The probable explanation of the origin of nerves from the neural crest has 

 already been briefly given (p. 262). It is that the neural crest represents 

 the original lateral borders of the nervous plate, and that, in the mechanical 

 folding of the nervous plate to form the cerebro-spinal canal, its two lateral 

 borders have become approximated in the median dorsal line to form the 

 neural crest. The subsequent shifting of the nerves I am unable to explain, 

 and the meaning of the transient longitudinal commissure connecting the 

 nerves is also unknown. The folding of the neural plate must have 

 extended to the region of the origin of the olfactory nerves, so that, as 

 just stated, there would be no special probability of the olfactory nerves 

 belonging to the same category as the other dorsal nerves from the fact of 

 their springing from the neural crest. 



FIG. 273. SECTION THROUGH THE BRAIN AND OL- 

 FACTORY ORGAN OF AN EMBRYO OF ScYLLIUM. (Modi- 

 fied from figures by Marshall and myself.) 



ch. cerebral hemispheres ; ol.v. olfactory vesicle ; 

 o//. olfactory pit; Sch. Schneiderian folds ; J. olfac- 

 tory nerve. The reference line has been accidentally 

 taken through the nerve to the brain ; pn. pineal 

 eland. 



