228 HOWARD AVERS. [Vol. IV. 



As has been demonstrated by several recent investigators, 

 the fore-brain and olfactory lobes are simple outgrowths of the 

 dorsal wall of the thalamencephalon. They appear early in the 

 development, it is true, but such early appearance is undoubtedly 

 a coenogenetic phenomenon. That there is a substantial agree- 

 ment between the Amphioxus brain, and the higher vertebrate 

 brain of this stage, is perfectly evident. There is always a thala- 

 mocoele bounded anteriorly by a lamina terminalis to mark the 

 boundary of the primitive end of the neural axis, and above, be- 

 hind and on either side of this primitive lamina terminalis, the 

 optic diverticula are given off from the brain. 



D. All the sense organs developed in connection with the anterior 

 end of the Amphioxus body are probably paired ; some of them 

 certainly are, e.g. the eye-spot. 



As I shall show in the next paragraph, the eye of Amphioxus 

 exhibits unmistakable traces of bilateral symmetry and a ten- 

 dency to develop into two pigmented areas in connection with 

 diverticula of the thalamocoele, so that here I need only mention 

 the fact, that I have been able to trace fibres from the olfactory 

 organ through the walls of the olfactive diverticulum or bulbus 

 olfactorius to both sides of the brain. Whatever this organ 

 may prove to be, on further investigation of its ontogeny it is 

 supplied from bilateral brain centres and affects both right and 

 left co-ordinating tracts. The organs of special sense developed 

 in the course of the two anterior pairs of nerves are certainly 

 bilaterally symmetrical structures. 



E. The eye-spot or eye of Amphioxus is the forerunner of the 

 vertebrate eye, and shows traces of several stages in the develop- 

 ment of the retina of higher forms. In itself it is not an organ 

 of sight, but a light-perceiving organ. 



After a careful study of the Amphioxus eye-spot, and related 

 structures, I have become convinced that this animal presents us 

 with the earliest stage in the phylogenetic development of the 

 vertebrate eye. 



As is well known, this eye-spot, which is considered a pigment 

 spot, lies across the anterior end of the neural axis in the ante- 

 rior end or wall of the brain, i.e. in the lamina terminalis, in an 

 extended sense. It is not widely known, however, that this 



