1879.] 



and Olfactory Organ of Vertebrates. 



325 



The Development of the Olfactory Nerve. 



For the sake of clearness the more important conclusions are stated 

 in the form of propositions : — 



a. The olfactory nerves do not arise from the cerebral hemispheres, but 

 from the single unpaired forebrain. 



In chick embryos of about the fiftieth hour, or a little older, the 

 olfactory nerves can be clearly recognized arising from the upper 

 part of the sides of the forebrain. At this stage there is no trace 

 whatever of the cerebral hemispheres. The olfactory nerves then 

 come into existence before the cerebral hemispheres, and therefore 

 cannot be derived from them. The hemispheres are developed in the 

 chick as lateral outgrowths from the upper part of the forebrain ; at 

 first the olfactory nerves have no connexion with them, beyond that of 

 close proximity ; but very soon the hemispheres, by their rapid growth 

 forwards, drive the nerves down to the base of the brain, and so make 

 the nerves appear to arise from their under and anterior part. 



This account is confirmed in all points by observations on duck 

 embryos, which show clearly that the connexion of the olfactory 

 nerves with the cerebral hemispheres is not of a primary but of a 

 secondary or adaptative nature. 



In the dogfish (Scijllium) the forebrain is, as has been already 

 shown by Balfour,* single and unpaired up to stage 0, presenting 

 till then no trace whatever of a division into cerebral hemispheres : 

 the olfactory nerves are, however, well developed structures by 

 stage M; at which period they can be seen, in transverse sections 

 through the anterior part of the head, arising from the upper part of 

 the sides of the forebrain and running downwards to the olfactory 

 pits. The nerves can be recognized, though with less distinctness, 

 at still earlier stages. 



The olfactory nerves of the salmon and of the trout can, in a 

 similar manner, be identified before the cerebral hemispheres have 

 come into existence ; and the same statement applies to the axolotl. 



b. There is no trace of an olfactory lobe in the early singes of develop- 

 ment of the olfactory nerve. 



Since the olfactory lobes are commonly described as ^'hollow out- 

 growths of the cerebral hemispheres," and the olfactory nerves have 

 just been shown to arise quite independently of the cerebral hemi- 

 spheres, this second proposition is in reality already proved by the 

 first. However, as the existence of olfactory lobes has been supposed 

 to separate the olfactory from the other cranial nerves, it becomes 

 necessary to investigate carefully the time and conditions of their 

 appearance. 



i In the chick the olfactory nerve is in its early stages solid, and 

 * " Elasmobranch Fishes," p. 178. 



