THE SENSE-ORGANS 499 



order to stop possible light impressions from coming from with- 

 out the eyeball, i.e., the natural direction for the receptive 

 cells. 



The necessity of this arrangement becomes clear to anyone 

 who has followed the foldings of the embryonic layers, yet 

 there is scarcely anything in vertebrate construction that seems 

 a greater mechanical mistake, although there are many others, 

 like the appendix and the inguinal canal in man, where a lesser 

 error involves far more serious consequences. This error in 

 the arrangement of the retina, however, becomes still more ap- 

 parent when a comparison is made with the structurally sim- 

 ilar eye of the cephalopod molluscs (squid, devil-fish, etc.), in 

 which the retina is developed directly from the surface ecto- 

 derm and is placed in the natural way, with the terminal cells 

 lining its interior and the optic nerve entering it from behind. 

 Notwithstanding the fundamental differences in developrhent 

 between this eye and that of vertebrates, the final results, when 

 compared part by part, are marvelously similar, and the adult 

 eye of each is furnished with retina and crystalline lens, iris 

 and cornea. This case is thus one of the best examples of what 

 Mr. Darwin termed " analogical resemblances " ; that is, the 

 production of a similar organ in two unrelated forms and often 

 from entirely different starting points, not through any genetic 

 connection, but because of the same environmental influences, 

 which give rise to the same necessities. 



In its histological structure the vertebrate retina shows some 

 similarity to other well-developed portions of the brain, and ex- 

 hibits several layers of cells, connected with one another by 

 branching processes which interlace and thus continue the com- 

 munication from one to another. At the exact focal center of 

 the lens all but the terminal sense-cells disappear, and produce 

 a small depressed area, the area centralis. This is often in the 

 form of a circular pit, fovea centralis, but may be oval, or in 

 the form of a broad band or streak. It is, however, not always 

 depressed, and . may be entirely wanting. These variations 

 seem to bear little or no relation to phylogeny, since a fovea is 

 present in some fishes and in most Sauropsida, while the area 



