THE AMEBIC AN NATURALIST [VOL. XLII 



with the skin. Thus, excepting the eyes, all the more ob- 

 vious vertebrate sense organs show immediate and 

 simple relations with the external layer, the skin. 



From this standpoint the lateral eyes of the vertebrate 

 are by no means as simple as the other vertebrate sense 

 organs. As has long been known the vertebrate retina, 

 the essential nervous portion of the eye, does not arise 

 directly from the external layer of the animal, but is an 

 outgrowth from the brain. And though the brain arises 

 by involution from the ectoderm and the retina may there- 

 fore be said to come directly from that layer, the fact that 

 the retina does not precede the brain in its development 

 but at best differentiates at the same time with the brain, 

 leaves the question of the exact origin of the retina in un- 

 certainty. 



Not only is the vertebrate retina, unlike other verte- 

 brate sense organs, not obviously connected with the 

 external ectoderm, but it is also peculiar in the arrange- 

 ment of its receptive cells, the rod- and cone-cells. In 

 the great majority of sense organs, the receptive mech- 

 anism is made up of cells with very marked polarity in 

 that one end of the cell is differentiated as a receiving 

 structure and is directed toward the impinging stimulus, 

 and the other end either tapers into a fibrous process, the 

 nerve fiber, or is in intimate relation with nerve fibers 

 from other cell bodies. In other words, the sense cells 

 are differentiated in that one end serves for reception, 

 and the other for transmission, and the cell is so placed 

 that the receptive end is nearer the source of the stimulus 

 than the transmitting one. In the vertebrate retina this 

 condition is reversed. The light that enters the eye falls 

 first upon the transmitting ends of the sense cells and 

 only after it has passed through these cells does it reach 

 the receptive end. The vertebrate retina is, therefore, 

 appropriately described as an inverted sense 

 Thus the vertebrate eye departs in two important 

 ulars from the other vertebrate sense organs: first, 

 sensory elements do not arise in any simple or direct 



organ. 



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