OTHER GRADIENT THEORIES 173 



mined with reference to a first-formed dominant region which 

 also has a morphogenetic or organizing effect, and that the 

 parts and organs involved in an active gradient system are 

 bound up together in a single physiological field, so that 

 alterations in one part will necessitate correlative alterations 

 elsewhere. 



In connexion with the persistence of gradient effects through- 

 out life, we may refer to some further examples not cited by 

 Child. Apart from the continuous growth-gradients in single 

 organs, in body-regions and in the body as a whole which 

 we have discussed in this book, we have also discontinuous 

 growth-effects which we can only understand on the basis of 

 continuous underlying gradients (p. 152). And we have also 

 gradients affecting the rate of growth or regeneration of 

 epidermal structures such as feathers and hair (p. 100). 



Clausen (1929), by grafting methods, has shown the exist- 

 ence in the tadpole tail of a gradient in susceptibility to the 

 autolysing agencies which operate at metamorphosis : skin 

 and muscle grafts from the anterior regions of the tail when 

 transplanted to the back undergo more rapid histolysis during 

 metamorphosis than do similar grafts from more posterior 

 situations. This is a significant fact, for it cannot well have 

 any particular functional or adaptive significance in relation 

 to the metamorphic process, and the difference, like that be- 

 tween the two horns of the rhinoceros, merely acts as an 

 indicator for the existence of some fundamental inherent 

 property of the organism. 



To take quite another example, Alverdes has demonstrated 

 the existence of graded peristaltic activity in regions of the 

 mammalian gut, showing that physiological gradients may exist 

 even where no morphological differentiation is visible. 



A gradient as regards regeneration is also seen in the results 

 of von Ubisch. From his data (1915) on regeneration of limbs 

 in the insect larva Cloe diptera, we can calculate that the 

 length of the femur of regenerated limbs after one moult- 

 interval is 44-2 per cent, of the length of the normal femur 

 for the fore-limb, 40-6 per cent, for the middle limb, and 

 367 per cent, for the hind limb. This differs in sign from that 

 for growth in Sphodromantis (p. 135), but the data are not 

 wholly comparable. 



In a later paper (1922) he refers to the fact that Przibram 

 (1919) and Krizenecki (1917) obtained a gradient of opposite 

 sign for the regeneration of the limbs of mantids and meal- 

 worms and thinks that his work on Cloe might have given 



