236 Journal of Comparative Neurology and Psychology. 



attachment of the optic stalk becomes carried downward to the 

 ventral side of the brain." The figures clearly illustrate this as 

 a progression of the optic stalk from the dorsal to the ventral 

 side of the brain. When this is taken together with what is 

 said below about the optic tract I think it will be seen to have 

 considerable significance. At all events the idea that the eye 

 is a dorsal structure involving the dorso-lateral wall of the brain, 

 rather than a ventral one, is of the first importance, if true. I 

 believe that the ontogeny favors this view, although the op- 

 posite has been universally held. In correlation with this I wish 

 to point to an important fact in the gross anatomy of the fore- 

 brain. In all lower vertebrates there is between the prosen- 

 cephalon and diencephalon a very slender region made up chiefly 

 of fiber tracts. This slender portion is due to a very deep in- 

 dentation of the dorsal-lateral wall of the brain above the optic 

 chiasma, and the choroid plexus, is widest at this point. This 

 slender region becomes less noticeable in higher vertebrates, 

 where the fore-brain grows larger and more rapidly, but it is 

 clearly seen in embryos and is marked in the adult by the wide 

 choroid plexus. The facts suggest that the nervous material of 

 the dorso lateral wall of the brain tube has been withdrawn in 

 this region and its place is filled merely by a thin membrane. 

 I believe that this is due to the formation of the optic vesicle 

 and that the nerve centers which are recognized as constituting 

 the retina have been derived from the brain at this point, namely 

 in the caudal portion of the fore-brain dorsal to the permanent 

 attachment of the optic stalk. 



If we turn to the central relations we shall find a number 

 of very significant facts which explain this relation of the optic 

 vesicle to the brain wall. The optic tracts form a decussation 

 in the ventral wall of the brain which in lower vertebrates is 

 complete and simple. These tracts then enter the optic lobes 

 which were primitively their sole place of ending. These lobes 

 are also the place of ending of the internal arcuate fibers (tractus 

 bulbo-tectalis) from the general and special cutaneous centers in 

 the medulla oblongata. These tracts from the retina and from 

 the cutaneous centers constitute primarily the only afferent 



