THE VERTEBRATE EYE 237 



1. The ICHTHYOPSIDA — Fishes and Amphibians, the primary habitat of 

 which is water — completely so in the case of the first and developmentally so in 

 the second. Although the eyes of adult Amphibians show many terrestrial 

 adaptations, the larval stage is spent in water and the adjustments for aerial 

 vision are added to the general plan of the aquatic eye. 



2. The SAUROPSIDA — Reptiles and Birds which, despite the difference in 

 their external appearance, show many close structural affinities. In them the 

 eyes have become completely adapted to aerial vision. 



3. The MAMMALIA, in which the eye, starting from a primitive reptilian 

 source, has developed along separate lines adapting itself to almost every 

 environmental habitat — including a return to aquatic vision. 



THE PHYLOGENY OF THE VERTEBRATE EYE 



We have already seen that the eyes of Invertebrates are developed 

 from the surface ectoderm and that the visual cells are connected to 

 the nervous system secondarily ; the eyes of Vertebrates, on the other 

 hand, arise from the neural ectoderm. It is true that the neural 

 ectoderm itself is ultimately derived as an infolding from the surface 

 layer, but the cerebral eye of Vertebrates indicates a major evolu- 

 tionary step affording the sentient layer of cells all the opportunities 

 for the pluripotential differentiation characteristic of the central 

 nervous system of which in every sense it forms an integral part. An 

 apparatus capable of subserving a highly developed sense of vision 

 depends no less on the efficiency of its central nervous representation 

 which interprets its images than on the peripheral sensory apparatus 

 which receives and resolves them. Moreover, an endoneural receptor 

 immune because of its position to other stimuli, mechanical or 

 chemical, can evolve a delicacy of response without danger of false 

 alarms that could not be attained by an organ exposed on the surface. 

 The significance of the origin of the vertebrate eye is thus apparent ; 

 the process is essentially the same as in Invertebrates, both the eye and 

 the central nervous system being ectodermal, but in the latter the eye 

 has evolved from the surface ectoderm primarily, in Vertebrates it is 

 secondarily derived.^ 



The curious thing, however, about the evolution of the vertebrate 

 eye is the apparent suddenness of its appearance and the elaboration of 

 its structures in its earliest known stages. There is no long evolutionary 

 story as we have seen among invertebrate eyes whereby an intracellular 

 organelle passes into a unicellular and then a multicellular eye, attaining 

 by trial and error along different routes an ever-increasing degree of 

 complexity. Within the vertebrate phylum the eye shows no progress 

 of increasing differentiation and perfection as is seen in the brain, the 



^ It is to be noted that the sensory cells in the epidermis of the tail of the ammocoete 

 larva of the lamprey are probably light-sensitive (Steven, 1950-51) ; they resemble 

 the apolar light cells seen in some worms (Lumbricu.s) and molhiscs (Mya) (p. l.*^!). 

 This is the only instance of the occurrence among Vertebrates of the primitive light 

 cells characteristic of Invertebrates, and is analogous (perhaps) with the cells of Joseph 

 seen in the integument of Amphioxus (p. 229). 



