No. 501] THE ORIGIN OF VERTEBRATE EYES 609 



region of the nerve tube. Furthermore in the brain 

 region the development of the musculature for the gills 

 and jaws together with the growth of the cranium as a 

 support for this musculature must have cut off the light 

 from below. Hence the natural growth of the body of 

 the fish would tend to restrict the occurrence of the visual 

 cells to the sides of the anterior end of the nerve tube, 

 the regions from which the eyes of the vertebrates de- 

 velop. It is in this partial envelopment of the nerve 

 tube with somewhat opaque tissue that, in my opinion, 

 has limited the eyes of vertebrates to their present posi- 

 tions. 



An answer to the second question raised, namely, why 

 is the vertebrate retina inverted, is found, I believe, in 

 the structure of just such direction eyes as amphioxus 

 possesses. The visual cell in each of these eyes has its 

 receptive end buried in the depths of its pigment cup and 

 its fibrous end directed outward through the open mouth 

 of this cup. As the effective light necessarily enters the 

 cup through its mouth, the visual cell is inverted in its 

 relation to this stimulus. If then we imagine a retina to 

 be formed from a great aggregation of individual direc- 

 tion eyes of the type found in amphioxus, it follows that 

 the receptive elements of such a retina would be inverted. 

 This, in my opinion, is the reason for the inversion of the 

 receptive cells in the retina. The vertebrate retina then 

 is fundamentally unlike the other vertebrate sense or- 

 gans in that it is not directly derived from the external 

 ectoderm, but is developed as a part of the central ner- 

 vous system, and its inversion is dependent upon its 

 origin from an aggregation of direction eyes each one of 

 which was of necessity inverted from the beginning. 



