571 



bulum. The latter organ in Amphioxus and, at all events, in the 

 lower Craniata is evidently of great importance; and primitively be- 

 sides being visual it had evidently other functions as well. It is 

 possible that the neuropore persisted for a long period as such in a 

 functional condition for the purpose of permitting the water to reach 

 this organ and the olfactory prosomere, the water escaping posteriorly 

 in the region of the anterior rhombomeres. Later with the increase in 

 size of the head region in front and the bending of that region ventral- 

 wards the infundibulum gained its relationship with the exterior from 

 beneath entering for the purpose into close association with the sto- 

 modeal ectoderm. The lips of the neuropore would also during this 

 period be becoming more and more olfactory, a function which it prob- 

 ably shared with the infundibulum, and later the nasal epithelium 

 would be shut out from the prosomere. 



The fragmentary evidence we possess indicates that the lateral 

 eyes have also originated later from the dorso - lateral walls of the 

 optic segment in connexion with the now dorso - lateral optic ganglia. 

 There is no evidence at all events of paired eyes below the Craniata 

 in the Chordate series. The conclusion therefore is that an eye of 

 the same nature as the pineal arose as soon as the expansion of the 

 second prosomere to the surface brought the pineal eye into a dorsal 

 position. But the development in this case would necessarily result 

 in the retinal elements being reversed. The two pairs of eyes would 

 have been functional until the better placed lateral eyes were fully 

 established. The median eyes belong therefore to a region external 

 to the paired eyes, and the latter are in the Craniata caenogenetically 

 earlier originated. Furthermore it is evident from this account that 

 with the bending downwards of the primitive fore brain the de- 

 veloping eye on each side would come into association with the epi- 

 branchial ganglion of the first cleft at a time when the degeneration 

 of its visceral nerve had already taken place, and thus the ganglion 

 may have provided the materials for the lens. 



The third prosomere is primitively oral in position, and in asso- 

 ciation with' it and the following encephalomeres 16 ganglia and vis- 

 ceral nerves are accounted for, and this region was also originally 

 provided with 15 somites and ventral nerves. 



There is no need to state again the changes which these nerves 

 and somites have undergone in the evolution of the Craniates. The 

 results have already been set forth in the preceding pages and are 

 given graphically in Fig. 7 which should be compared with Fig. 6. 



The post-stomial part of the central nervous system apparently 



