304 DR. A. MILNES MARSHALL. 



the olfactory nerve ; this olfactory lobe grows very rapidly, 

 and soon attains a large size, while the olfactory nerve re- 

 mains almost stationary ; the nerve is at first connected with 

 the apex of the olfactory lobe, but subsequently mounts 

 somewhat on to its dorsal surface ; finally, the olfactory nerve, 

 throughout its development, agrees closely in histological 

 characters and in the changes which it undergoes with 

 the other cranial nerves. 



Balfour has given a somewhat different account of the 

 development of the olfactory nerve. After noticing that the 

 olfactory lobes first arise during the stage o, he says : — 

 ''From the peripheral end of each olfactory lobe a nerve, 

 similar in its histological constitution to any other cranial 

 nerve, makes its appearance ; this divides into a number of 

 branches, one of which passes into the connective tissue 

 between the two layers of epithelium in each Schneiderian 

 fold. On the root of this nerve there is a large development 

 of ganglionic cells. I have not definitely observed its origin, 

 but have no reason to doubt that it is a direct outgrowth 

 from the olfactory lobe, exactly similar in its mode of deve- 

 lopment to any other nerve of the body."^ A little furtlier 

 on he remarks : "Even the few preparations of which I have 

 given figures appear to me to prove that . . . from the 

 (olfactory) bulb a nerve grows out which has a centrifugal 

 growth like other nerves of the body, and places the central 

 olfactory lobe in communication with the peripheral olfactory 

 sack."2 



The differences between this account and my own are suffi- 

 ciently obvious. According to Balfour, the olfactory lobe 

 exists before the olfactory nerve, and the nerve is a " direct 

 outgrowth from the olfactory lobe." A minor point of 

 difference is that, according to Balfour, the connection be- 

 tween the olfactory nerve and the olfactory pit is not acquired 

 till towards the end of stage o. I believe, however, that these 

 differences are due to Balfour having overlooked the exist- 

 ence of the olfactory nerve during its early stages. The 

 first stage at which he has described the olfactory nerve is 

 that which I have represented in fig. 21,^ while the specimens 

 I have figured (figs. 19 and 20) appear to me to prove indis- 

 putably the existence of the olfactory nerve at a much 

 earlier period, and the connection between the olfactory nerve 



1 Op. cit., p. 178. 



2 Op. cit., p. 183. 



3 The section from which this figure is drawn is one of the same series, 

 if not the identical specimen, as that described by Balfour, and figured by 

 liini in n. XA^ fig. 2. 



