PARKER. — THE SENSORY REACTIONS OF AMPHIOXUS. 445 



functional on the exterior of the vertebrate ancestor before this animal 

 had an infolded central nervous system, and that in the course of its 

 differentiation it had passed as a functional eye into the deeper parts 

 of the head and out to the surface again, a process not so difficult to 

 understand when it is kept in mind that the bodies of many tunicates 

 and of amphioxus are relatively transparent. Essentially the same 

 explanation has been brought forward recently by Jelgersma 06), who 

 believes that the eye in its transition between its supposed place of 

 origin in the skin and its final position in the vertebrate head is well 

 represented by the eye of the larval tunicates. Boveri (: 04) has called 

 attention to the strong probability that the lateral eye has been derived 

 from photoreceptors in the central nervous system, and has pointed out 

 that the eye-cups of amphioxus are the probable source. He has not, 

 however, attempted to trace these eye-cups back, as Jelgersma (:06, 

 p. 393) has done, to a possible origin in the skin, but implies that they 

 may have arisen in place. 



Although I believe that the explanation first advanced by Balfour as 

 to the origin of the lateral eyes of vertebrates has some truth in it, 

 there are certain aspects of it which in view of the present investiga- 

 tions need further consideration. Its first assumption is that the skin 

 of the ancestral vertebrate contained photoreceptors. The fact already 

 mentioned, that the skin of some amphibians and fishes, particularly 

 ammocoetes (Parker, :03s :05b), {g gQ supplied, would lead to the ex- 

 pectation that the skin of amphioxus would also contain such organs. 

 My own studies have given no grounds for this belief, and, though I 

 have not been able conclusively to prove the contrary, the evidence 

 seems to favor the idea that the skin of amphioxus is not sensitive to 

 light. As nothing is known, so far as I am aware, of the condition of 

 the skin in this respect in tunicates, adult or young, the belief that the 

 skin of the ancestral vertebrate contained photoreceptors must remain 

 a pure hypothesis, and it is conceivable that the photoreceptors of the 

 vertebrate eye may have arisen, not in the skin before the central 

 nervous system was differentiated, as suggested by Balfour and by 

 Jelgersma, but, as intimated by Boveri, fi-om the cells of the central 

 nervous system itself, in positions much as we find them now in 

 amphioxus. 



The assumption of an external origin for the vertebrate photo- 

 receptors is helpful only in that it appears to offer an explanation of 

 the inverted positions of the rods and cones in the vertebrate retina. 

 But this explanation requires that from the time the photoreceptors 

 were formed in the skin till they made a part of an organized retina, 

 they should occupy the morphologically outermost portion of the cellular 



