314 THE PR/KORAL LOBE. 



tions of part of the epiblast covering it. Is such an element to 

 be recognized in the head of the Chordata ? From a superficial 

 examination of Amphioxus the answer would undoubtedly be 

 no ; but then it has to be borne in mind that Amphioxus, in 

 correlation with its habit of burying itself in sand, is especially 

 degenerate in the development of its sense-organs ; so that it is 

 not difficult to believe that its praeoral lobe may have become so 

 reduced as not to be recognizable. In the true Vertebrata there 

 is a portion of the head which has undoubtedly many features of 

 the praeoral lobe in the types already alluded to, viz. the part 

 containing the cerebral hemispheres and the thalamencephalon. 

 If there is any part of the brain homologous with the supra- 

 cesophageal ganglia of the Invertebrates, and it is difficult to 

 believe there is not such a part, it must be part of, or contain, 

 the fore-brain. The fore-brain resembles the supraoesophageal 

 ganglia in being intimately connected in its development with 

 the optic organs, and in supplying with nerves only organs of 

 sense. Its connection with the olfactory organs is an argument 

 in the same direction. Even in Amphioxus there is a small 

 bulb at the end of the nervous tube supplying what is very 

 probably the homologue of the olfactory organ of the Vertebrata ; 

 and it is quite possible that this bulb is the reduced rudiment of 

 what forms the fore-brain in the Vertebrata. 



The evidence at our disposal appears to me to indicate that 

 the third nerve belongs to the cranio-spinal series of segmental 

 nerves, while the optic and olfactory nerves appear to me 

 equally clearly not to belong to this series 1 . The mid-brain, as 

 giving origin to the third nerve, would appear not to have been 

 part of the ganglion of the prseoral lobe. 



These considerations indicate with fair probability that the 

 part of the head containing the fore-brain is the equivalent of the 

 praeoral lobe of many Invertebrate forms ; and the primitive 

 position of the Vertebrate mouth on the ventral side of the head 

 affords a distinct support for this view. It must however be 

 admitted that this part of the head is not sharply separated in 

 development from that behind ; and, though the fore-brain is 



1 Marshall, in his valuable paper on the development of the olfactory organ, takes 

 a very different view of this subject. For a discussion of this view I must refer the 

 reader to the chapter on the nervous system. 



