446 PROCEEDINGS OF THE AMERICAN ACADEMY. 



layer in which they were imbedded and that the individual photo- 

 receptors should be so oriented that their sensory ends would be toward 

 the morphologically outer surface of this layer and their nervous ends 

 away from it. In amphioxus it is true that the photoreceptors lie near 

 the morphologically outer surface (the surface of the central canal), 

 but their orientation is by no means constant in relation to this surface. 

 In some the sensory ends point toward this surface, but in most such 

 is not the case, and in a few they may even point away from this sur- 

 face. It therefore seems to me obviously impossible to explain the 

 orientation of the retinal rods and cones as transferred from the skin 

 to the retina through a series of stages in one of which as much free- 

 dom of position is shown as among the photoreceptors of amphioxus. 

 Nor, as Metcalf (:06, p. 528) has pointed out, is the condition more 

 favorable in the larvae of the tunicates, for here the photoreceptor 

 cluster in the brain is so large compared with the thickness of the 

 cellular wall in which it is imbedded (Froriep, : 06, p. 145) that its 

 orientation is no more related to the morphologically outer surface of 

 the wall than that of the eye-cups of amphioxus is. For these reasons 

 I believe that the inversion of the vertebrate rods and cones in relation 

 to the light is not due to their origin from definitely oriented external 

 photoreceptors, and since there is no positive evidence of the existence 

 of these receptors in the skins of animals that may fairly represent an- 

 cestors of the vertebrates, it seems to me that we are not warranted in 

 assuming their presence at all. I therefore agree with Boveri in 

 believing that the photoreceptors of vertebrates have arisen in the 

 central nervous system and not in the skin, as assumed by Balfour and 

 by Jelgersma. 



If the unusual position and orientation of the rods and cones in 

 the vertebrate retina are not due to the origin of these bodies from ex- 

 ternal photoreceptors, how then are these peculiarities to be accounted 

 for? The position of the photoreceptor near the central canal is due 

 in my opinion to the method of growth of the nerve-tube, for the epi- 

 thelium surrounding the central canal is the source of the various cells 

 in the wall of the tube. When, therefore, a new type of cell, like the 

 photoreceptor, appears, it would be natural to expect it to arise from 

 this undifferentiated material, and, in my opinion, the photoreceptors 

 of amphioxus and of the tunicate larvae are in their position of origin. 

 This position is retained by their derivatives the rod- and cone-cells. 



The very exact orientation of the rods and cones involves factors 

 quite different from those that govern their general position. The 

 eye-cups of amphioxus show only a very slight degree of orientation, 

 but so far as this goes, it is correlated with habit, in that the majority 



