Eycleshymer, Reactions of Necturus. 307 



until the writer ('93) showed that in Rana palustris the Anlagen 

 of the optic vesicles not only appear as a pair of pigmented areas, 

 but that these areas are made up of pigmented columnar cells so 

 different from the cells in the remainder of the neural plate that 

 there could be no reasonable doubt of their being specially differ- 

 entiated areas. By following these areas step by step during the 

 period of closure of the neural folds it was definitely established 

 that these areas formed the bases of the future retinae. 



Shortly after the pubhcation of the writer's observations LocY 

 ('93) found a series of depressions in the unclosed neural plate 

 of certain Elasmobranchs which he thought represented paired 

 sensory structures, probably, of a visual character. 



In a word, it may be said that the evidence has been slowly 

 accumulating from the morphological side in support of the 

 hypothesis that the retina belongs to the cutaneous sensory system. 



The evidence from the physiological side is equally confirma- 

 tory. Parker ('05, p. 419) who has recently carefully reviewed the 

 hterature states that "This sensitiveness of the vertebrate skin 

 to light is probably a remnant of that primitive condition from 

 which the lateral retinas were derived, and possibly served as a 

 basis from which the temperature terminals of the skin in the 

 higher vertebrates developed." 



In conclusion, then, one may say all the evidence goes to show, 

 as Johnston ('05, p. 241) has well stated, that "the retina belongs 

 morphologically, as well as physiologically, to the cutaneous 

 sensory system." 



BIBLIOGRAPHY. 



Balfour, F. M. 



'85. A Treatise on Comparative Embryology. London, MacmiUan & Co. 

 Beard, J. 



'88. The Old Mouth and the New. Anat. Anzeiger, Bd. 3, pp. 15-23. 

 Dubois, R. 



'90. Sur la perception des radiations lumineuses par la peau, chez les Protees aveugles des 

 grottes'de la Carniole. Comp. rend. Acad. Sci., Paris, Tom. no, pp. 358-361. 

 Etcleshymer, a. C. 



'93. The Development of the Optic Vesicles in Amphibia. Journ. MorphoL, vol. S,Y>p- 189- 

 194. 

 Graber. 



'84. Grundlinien zur Erforschung des Helligkeits und Farbensinnes der Thiere, Prag, pp. 

 1-322. 

 Hbape, W. 



'84. The Development of the Mole. Stud. Morp. Lab., Cambridge, Eng. Vol. 2, pp. 30-69. 



