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Dr. A. M. Marshall on the Olfactory Nerve [Feb. 13, 



from a histological point of view differs in no appreciable respect 

 from the other cranial nerves at corresponding stages of their develop- 

 ment. At the end of the sixth day of incubation the nerve, which is 

 now of some length, has acquired its secondary connexion with the 

 cerebral hemisphere in the manner described above ; yet the nerve is 

 still solid along its whole length, and presents no trace whatever of 

 an olfactory lobe, or hollow outgrowth from the brain. By the end 

 of the seventh day a very small conical pit is visible in the wall of the 

 cerebral hemisphere at the point of origin of the olfactory nerve. 

 This pit, which is the earliest rudiment of the olfactory lobe, is formed 

 almost entirely at the expense of the inner wall of the hemisphere, so 

 that there is hardly any corresponding projection on the outside of 

 the brain. 



The development of the olfactory lobe in the dogfish closely 

 resembles that in the chick : at stage M there is no trace whatever of 

 a lobe, though the olfactory nerves are large and conspicuous struc- 

 tures. At a stage a little younger than Balfour's stage 0, the first 

 rudiment of an olfactory lobe appears, as a slight lateral bulging of 

 the side of the forebrain, at the point of origin of the olfactory nerve : 

 this increases rapidly, much more so indeed than the nerve itself ; by 

 stage it is a tolerably prominent structure, and in the later stages 

 it becomes considerably larger than the nerve proper.* 



Stage in the development of a dogfish embryo corresponds to 

 about the sixth day in the chick, so that there is a close agreement in 

 time as well as in mode of development of the olfactory lobe in these 

 two types. In the dogfish, however, the olfactory lobes appear before 

 the cerebral hemispheres are differentiated, and consequently arise 

 ' from the forebrain ; while in the chick the hemispheres are developed 

 rather earlier, and the olfactory lobes arise as direct outgrowths from 

 them, and not from the original forebrain. 



In the salmon and trout, from the earliest period at which the 

 existence of an olfactory nerve can be recognized up to the time of 

 hatching, and indeed for some little time afterwards> there is no trace 

 of an olfactory lobe. 



The existence of an olfactory nerve without any trace of an olfactory 

 lobe has also been established in the earlier embryonic stages of the 

 axolotl, of the common frog, and of the green lizard. 



The olfactory nerve of an adult vertebrate is commonly described 

 as consisting of three parts, a proximal element or olfactory tract, an 

 intermediate olfactory bulh, and a distal olfactory nerve proper, the two 

 former of these corresponding to the olfactory lobe or vesicle of the 

 embryo. From the descriptions given above it would appear that the 



* Cf. Balfour, op. cit., p. 178, and Plate 15, figs. 2 and 8a. Balfour has not ob- 

 sei'ved the olfactory nerves earlier than stage O, and therefore describes them as 

 outgrowths from the olfactory lobes. 



