232 HOWARD AVERS. [Vol. IV. 



The primitive sense cells of the ectoderm transmitted through 

 the nerve fibres of the first pair of nerves general impressions, 

 from which those percipient elements of the brain associated 

 with pigment cells selected vibrations of those wave lengths 

 productive of the sensations of light and heat. The percip- 

 ient elements not associated with pigment cells selected and 

 were affected by only such stimuli as gave rise to other sensa- 

 tions, taste, smell, touch, etc. 



Now that we have seen how most of the structural details of 

 the brain form presented to us by all vertebrates higher than 

 Amphioxus may have arisen out of the simple Amphioxus type, 

 there remains for consideration a structure whose significance, 

 both morphological and physiological, is still a matter of dis- 

 cussion, viz., the hypophysis. So far as I have been able to 

 discover, there does not exist in Amphioxus the slightest trace 

 of such an organ ; but it is not difficult to see how such a struc- 

 ture could arise from the ventral portion of the thalamocoele, in 

 connection with the further development of the mouth and its 

 sense organs. For the present I am not in position to say 

 more of this structure, than that the hypophysis in Ammocoetes 

 is intimately related with the olfactive area, and was probably an 

 organ of taste. 



We have then, in Amphioxus, all the stages in the develop- 

 ment of the paired eyes of vertebrates (as well as of the pineal 

 or unpaired eye) demanded by the theoretical considerations, 

 save the final ones of the formation of the completed optic vesi- 

 cles and lenses. The completion of these stages occurred 

 somewhere in the phyletic series between the Amphioxus condi- 

 tion and the Cyclostome condition. In the Cyclostomata 

 (Ammocoetes) the optic vesicles grow out only slowly, and after 

 the animal has passed the Amphioxus stage. 



It should not be forgotten that the optic vesicle, as it pushes 

 out from the brain, presents its anterior hemisphere to the exte- 

 rior ; and that in most vertebrates this half remains unpigmented, 

 while it is this anterior portion which is pigmented in Amphi- 

 oxus. Such must have been the primitive condition of the optic 

 vesicle however developed. 



From what has been said, it follows that there is no posterior 

 hemisphere developed, because the process never goes far enough 

 to form a spherical bulb, and an optic stalk. 



