2o8 COMPARATIVE PHYSIOLOGY 



experiments of Shearer have been cited in some detail. It 

 opens up the possibility of substituting for architectural 

 mnemonics quantitative experiment along two lines : (i) how 

 external agencies acting on the egg or embryo set up spatially 

 orientated differences in cellular oxidative or other processes ; 

 in this connection important w^ork on electrical gradients in 

 organisms has been done by Lund (1921-22) and Hyman and 

 Bellamy (1922). (2) How the structural features of isolated 

 tissues are affected by artificially induced differences in oxygen 

 consumption. In relation to the last issue papers by Huxley on 

 de- differentiation maybe mentioned for suggestive indications. 

 Let us now turn from the spatial aspect of individuation to 

 the mechanism which determines the orderly succession of 

 developmental stages. The nature of the issue is clearly 

 presented by reference to experiments of Uhlenhuth (1912-17) 

 who has studied the effects of grafting eyes and skin of larval 

 salamanders into individuals of different ages. His observa- 

 tions show conclusively that when such organs are transplanted 

 they assume the adult characteristics not at the time when their 

 original possessor attains maturity, but always when the animals 

 into which they have been grafted attain metamorphosis. 

 Hence for the development of, say, the adult skin characteristics 

 (^e.g. yellow pigment areas) there must be present something 

 which is normally produced at the time of metamorphosis 

 and is produced by the body as a whole or by some special 

 organ or organs. The nature of this factor is now clearly 

 established as regards the case selected. 



Animals which like Amphibia undergo a metamorphosis 

 are pecuUarly suitable for the study of the time factor in 

 development. Up to a certain point individuation proceeds 

 actively. It is then checked ; growth continues for a period 

 without much structural rearrangement. Then a second 

 phase of active structural differentiation is intercalated. In 

 the Anura this involves (i) closure of the gill clefts ; (2) resorp- 

 tion of the tail ; (3) full development of the limb rudiments. 

 InUrodeles the events are (i) resorption of the external gills ; 



(2) resorption of the dorsal fin and shedding of the larval skin ; 



(3) closure of the gill clefts. The Urodele larva has fully- 



