vi Journal of Comparative Neurology. 



says, " In mammals the optic stalk becomes converted into the optic 

 nerve by the transformation of its protoplasmic substance into reti- 

 form sustentacular tissue, and by the passage of nerve fibres through 

 its walls, the nerve fibers being protected and supported by the exter- 

 nal limiting membrane of the stalk, and by the reticular framework 

 formed by the modification of its walls, the transformation from the 

 stalk to the nerve being associated with the disappearance of the cav- 

 ity of the stalk." 



In further discussing the relation of the retina and optic nerve to 

 the brain. Dr. Robinson calls attention to the differences between the 

 retina and the other sense organs. The fact that the retina is an 

 evagination from the brain he regards as involving the absence of both 

 peripheral nerve and ganglion, external stimuli impinging directly 

 upon the central organs ; and from the fact that the optic evagination 

 takes place from the ventral part of the primary vesicle he is inclined 

 to look upon the retina as a modified basal ganglion and the optic nerve 

 as an association bundle bringing it into relation with other parts of 

 the brain. The facts, however, are capable of quite a different inter- 

 pretation in the light of the most recent researches. It will be remem- 

 bered that Eycleshymer and Locy have described protons of the optic 

 vesicles on the cephalic plate even before its invagination to form the 

 neural tube and that Locy recognizes behind these vesicles other ac- 

 cessory vesicles, one pair of which he regards as the protons of the 

 epiphysis. The elements of the retina therefore can be traced direct- 

 ly back to definite areas of ectoderm, like the cranial ganglia in gen- 

 eral and they cannot be compared with basal ganglia. It has been 

 suggested that we go a step farther and compare the cells of the gan- 

 glionic layer of the retina with the cells of the cranial ganglia and the 

 fibers of the optic nerve with root fibers. Thus the retina, though a 

 part of the brain, may at the same time contain structures which are 

 ordinarily found in peripheral ganglia. 



Final judgment on these questions is probably at present prema- 

 ture, but an interpretation along these lines seems to the writer to be 

 better in accord with the existing evidence than to regard the retina 

 as a basal ganglion. 



However clearly the facts of development may point to the com- 

 munity of origin of the optic apparatus and the other parts of the 

 brain, there still remain in the adult structures of the lower vertebrates 

 especially many points difficult of interpretation. Several compara- 

 tive studies of the optic nerves of vertebrates have been made. The 



