244 



THE EYE IN EVOLUTION 



be ependymal in origin, the receptor end being phylogenetically 

 homologous with the single cilium of an ependymal cell, was revived by 

 Leboucq (1909), a theory which was elaborated with great persuasive- 

 ness by Studnicka (1912-18), and subsequently supported by Walls 

 (1939) and Willmer (1953). In this view the phylogenetic homologue 

 of the vertebrate retina may be assumed to be the infundibular organ 

 of Amphioxus ^ ; but it must be remembered that any convincing 

 phylogenetic sequence connecting the two is lacking. 



Figs. 25.5 to 258. — Boveri's Conception of the Development of the 

 Vertebrate Eye from the Organs of Hesse of an Amphioxus- 

 LiKE Ancestor. 



Fig. 255. — Symmetrical arrangement 

 of the organs of Hesse with pigment 

 cells facing the central canal. 



Fig. 256. — Evagination of the canal 

 carrying with it the organs of Hesse. 



Fig. 257. — Invagination to form a 

 sensory and pigmented layer. 



Fig. 258. — Secondary invagination of 

 the lens vesicle (from Walls, after 

 Boveri). 



Such a development would not be unique since modified flagellated cells of 

 this type are also seen in other sensory organs such as the olfactory cells, the 

 hair cells of the labyrinth, the cells of the taste-buds and lateral line organs ; and 

 it is to be remembered that there is a considerable amount of evidence that the 

 ependymal cells in the diencephalic region retain some photosensory properties 

 in several species of Vertebrates - (von Frisch, 1911 ; Scharrer, 1928 ; Nowikoff, 

 1934 ; Young, 1935 ; Benoit, 1937 ; and others). 



Agreement on the ependymal origin of the visual receptors is, 

 however, by no means universal and many investigators, following 

 Schwalbe, believe that they are endoneural. Thus Boveri (1904) traced 

 their origin from the ganglion-like cells of Hesse in an Amphioxus-hke 

 ancestors In this view he was supported by Parker (1908-9), 

 1 p. 229. 2 p. 537. 3 p^ 230. 



