322 DR. A. MILNES MARSHALL. 



equivalent periods in the development of different verte- 

 brates. In the dogfish the olfactory lobe appears before the 

 division of the forebrain into cerebral hemispheres takes 

 place ; in the chick not till long after the appearance of 

 the cerebral hemispheres ; and in the salmon, at any rate, 

 not till after the time of hatching. 



Tliese facts would appear to indicate that the olfactory 

 lobe is to be viewed rather as an adult or adaptative than as 

 an embryonic or primitive structure ; a view that is materially 

 strengthened by the great variations in relative size of the 

 three elements of the olfactory nerve in various adult verte- 

 brates, to which attention has already been directed. 



One of the most remarkable features of the early stages of 

 development of all vertebrates, is the enormous preponderance 

 of the central nervous system to which at first everything 

 appears to be subordinate, and which exercises a most 

 important influence on the shape of the embryo. The rapid 

 growth of the neural surface causes the body to become 

 curved towards the ventral surface ; this curvature is naturally 

 most marked at the free extremities of the body, and at thehead 

 end is the main, if not the sole, cause of cranial flexure. 

 Owing to this cranial flexure the forebrain gets carried in 

 front of the olfactory sacs, and, consequently, the olfactory 

 nerves, which, as we have seen, acquire their connection with 

 the olfactory sacs at a very early age, at first run in a direc- 

 tion downwards and backwards. Vide figs. 2, 15, 17, 

 and 18. 



Having attained this enormous relative development the 

 nervous system stops for a while, and the face begins to 

 grow more rapidly, causing the so-called rectification of the 

 cranial flexure ; the olfactory sacs get carried further and 

 further forwards, so that the olfactory nerves, instead of 

 running downwards and backwards, now run directly down- 

 wards, or downwards and outwards as seen in fig. 33. The 

 face still continuing to grow rapidly, while the brain under- 

 goes little or no increase in length, the olfactory sacs get 

 carried in front of the forebrain, so that the olfactory nerves 

 now run downwards and forwards. A continuation of this 

 process carries the olfactory sacs still further forwards, to an 

 extent varying much in different vertebrates, so that the 

 olfactory nerves ultimately run directly forwards as in most 

 adult vertebrates. Vide fig. S6. 



All the nerves of the body undergo during their develop- 

 ment a considerable lengthening, owing to the gradual 

 separation of their central and peripheral ends ; but while 

 in the case of all the other nerves this is a gradual and con- 



