THE AMERICAN NATURALIST [Vol. XLII 



falls to the ground. I, therefore, do not believe that the 

 vertebrate retina is inverted because it has been inherited 

 from an integumentary source, and I agree with Boveri 

 that the condition in amphioxus removes all occasion for 

 the assumption that the vertebrate retina arose on the 

 exterior of an ancestral form and became deep-seated by 

 being involved in the developing central nervous system. 

 That the central organs should thus become a seat for the 

 origin of essentially sensory mechanisms is not surpris- 

 ing as long as such an appropriate stimulus as light can 

 in certain cases reach them. Even in our own bodies the 

 respiratory reflex is maintained through the direct stimu- 

 lation of the medulla by the blood that passes through 

 it, an operation the first step of which must be equivalent 

 to the process of sensory stimulation. 



I know of no reason for assuming that the visual appa- 

 ratus of amphioxus arose in any other than its present 

 position, and I believe there is good grounds, as Boveri 

 has pointed out, for the opinion that the visual cells of 

 amphioxus are the liomologues of the rod- and cone-cells 

 of the vertebrates. These visual cells then probably 

 represent the material out of which the vertebrate retina 

 has been made. If this is true why have the eyes of ver- 

 tebrates developed where they have and how does it come 

 that their retinas are inverted? 



An answer to the first of these questions can be found, 

 I believe, in the size of the vertebrate body as compared 

 with that of such an animal as amphioxus. In amphioxus 

 the body is so small and thin that it is sufficiently trans- 

 parent to allow light to penetrate it anywhere. Hence 

 visual organs may be functional at any point along the 

 length of its nerve tube. In most fishes, however, the in- 

 crease of size has involved such a development of the 

 body musculature that the posterior part of the nerve 

 tube, the spinal cord, is completely bnried in thick, almost 

 opaque tissue. Hence visual cells would no longer be 

 serviceable in such a region as this and they have doubt- 

 less thus been restricted to what has become the brain 



