306 DR. A. MILNES MARSHALL. 



it was taken being in unusually good preservation. The 

 section passes through the forebrain (/. h.), the olfactory pit 

 [olf.), and the olfactory nerve (i). From it we learn (1) 

 that the olfactory nerves exist prior to the cerebral hemi- 

 spheres, of which latter there is in this specimen no trace 

 whatever ; (2) that in this stage there is no indication what- 

 ever of an olfactory lobe ; (3) that the olfactory nerve is in 

 its early stages connected with the upper or dorsal part of 

 the side of the forebrain '^ (4) that the connection between 

 the olfactory nerve and olfactory pit is very early acquired ; 

 (5) that the olfactory nerve at this stage agrees closely in 

 histological characters with the corresponding stages of the 

 other cranial nerves, consisting almost entirely of roundish 

 or oval nucleated cells with few or no nerve fibres. 



This fio-ure may be advantageously compared with fig. 19, 

 which represents, as already described, a section through the 

 same region in a dogfish embryo at stage m. The resem- 

 blance between these two figures is indeed very striking, 

 and extends even to the minute histological details. I 

 would lay great stress on this resemblance, and submit that 

 this close correspondence, amounting almost to identity, in 

 the condition of the olfactory nerve at similar stages in two 

 vertebrates so Avidely separated as the chick and the dogfish, 

 affords very strong evidence in favour of the correctness of 

 my observations. Such differences as do exist are of very 

 minor importance. Apart from the slight difference in general 

 configuration, the most significant are the rather larger 

 relative size of the olfactory nerve and pit in the dogfish, 

 obviously correlated with their condition in the adult, and 

 the fact that in the duck the attachment of the olfactory 

 nerve is rather nearer to the summit of the forebrain than in 

 the corresponding stage of the dogfish. 



The appearance of the cerebral hemispheres towards the 

 close of the third day in the chick causes considerable altera- 

 tion in the position and relations of the olfactory nerves. 

 The hemispheres are lateral outgrowths of the forebrain, and 

 are from the first situated on the dorsal side of the roots of 

 the olfactory nerves. They grow forwards and upwards with 

 exceeding rapidity, and by so doing drive the olfactory nerves 

 down to the base of the brain, and so cause these nerves to 

 appear to arise from their under and anterior part ; a change 



1 Though the nerve is'iu close contact with the brain, the actual connec- 

 tion between the two is not seen in the specimen figured ; it is clearly visible 

 in one of the sections of the same series immediately adjacent, whiclij how- 

 ever, does not show the whole length of the nerve, and is, therefore, less 

 suitable for figuring. 



