FUNCTION AND FUNCTIONAL STIMULUS. 14! 



was that this degeneration was followed later by a complete regen- 

 eration of the transplanted eye, even in the dark. 



My experiments were conducted with the following considera- 

 tions in view. The retina is one of those structures which ac- 

 cording to Roux's definition should be called a functional struc- 

 ture. But in accepting this definition one need not necessarily 

 assume that these structures are the result of functional stimulus, 

 for such structures may have been predetermined in the embry- 

 onic stage. They develop independently of functional stimulus 

 until they reach the third period the so-called functional 

 period. 1 Functional structures can only exist during this period 

 if they are supplied with a sufficient amount of functional 

 stimulus; otherwise they undergo atrophy through inactivity. 

 From Roux's point of view it would appear probable that the 

 morphogenesis of the eye of Urodela is only partially determined 

 by inheritance, and in accordance with this determination it 

 would reach the same stage of development as that attained by 

 Proteus anguineus. The further development of the eye, com- 

 prising the formation of the rods and cones, that is the true 

 functional parts, would be brought about by the penetration 

 of the rays of light through the skin of the salamander, to its 

 ovaries, and would therefore be the outcome of functional 

 stimulus. Secerov 2 asserts that the skin of Salamandra macu- 

 losa permits of the penetration of approximately 1/173 of the 

 quantity of light in which the animal lives. 



The eyes of Proteus, which inhabits dark caves, must there- 

 fore remain in the primitive Proteus stage, according to the state- 

 ment of Kammerer, 3 who believes he has demonstrated that 

 further differentiation can only occur if Proteus be kept in 

 the light. Thus, he ascribes the process of full differentiation 



1 See, for example, Roux, "Die Entwicklungsmechanik, ein neuer Zweig der 

 biologischen Wissenschaft," Vortrage und Aufs. iiber Entwickl.-Mech. d. Organ- 

 ismen. 1905, Heft I., p. 94, note n. Also Roux, "Die vier Hauptperioden der 

 Ontogenese, sovvie das doppelte Bestimmtsein der organischen Gestaltungen," 

 Mitteilung der naturforschenden Gesdlschaft, Halle a.d.S., 1911, I., p. i. 



2 Secerov, S., "Die Umwelt des Keimplasmas. II. Der Lichtgenus im Sala- 

 mandra- Korper," Arch. f. Enlwcklungsm., 1912, XXXIII. , 682. 



3 Kammerer, P., "Experimente iiber Fortpflanzung, Farbe, Augen und Korper- 

 reduktion bei Proteus anguineus Laur. Zgl. Vererbung erzwungener Farbverander- 

 ungen." III. Mitteil., Arch. f. Entwicklungsm., 1912, XXXIII., 349. 



