366 GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 



already lost its regeneratory power, in which the leg consists simply 

 of a cartilaginous skeleton, muscles in process of differentiation, and 

 mesenchymatous connective tissue. It is necessary therefore to look 

 for another explanation. 



Grafting experiments have shown that tails and limbs of tadpoles, 

 transplanted on to adult frogs and then amputated, can regenerate 

 in their new position provided that the tadpole from which they 

 were taken had not already lost its regeneratory power. ^ The in- 

 ternal environment of the adult Anuran, therefore, does not provide 

 any factor specifically inimical to regeneration. Nor, on the other 

 hand, does the internal environment of the adult Urodele provide 

 any factor specifically helpful to regeneration, for a limb of a tad- 

 pole of the toad (Bufo) taken after the power of regeneration is lost, 

 grafted on to an adult Salamandra and amputated there, fails to 

 regenerate.^ 



The conclusion, is, therefore that loss of power to regenerate, 

 however it may originate, comes to operate regionally within the 

 fields themselves. It is not without interest to find that, in the 

 Urodeles, power to regenerate is eflFectively stopped if the whole 

 field is extirpated.^ This has been proved in respect of the snout, 

 the tail, and the limbs. 



§6 



Further evidence for the persistence of a total field is derived from 

 a study oi growth-gradients.^ 



In the first place the relative growth of parts, including the 

 phenomena seen in their regeneration, is regulated with reference 

 to a "growth-equilibrium" which concerns the organism as a 

 whole. The precise size of any part at any time depends on a 

 partition-coefficient of material as between the part and the rest 

 of the body (i.e. all the other parts). The value of this growth- 

 coefficient differs for different parts of the body, and depends 

 primarily on factors inherent in the tissues of the organ. If the 

 growth-coefficient is above unity, the part will increase in relative 

 size (positive heterogony) ; if below unity, it will decrease (negative 



^ Naville, 1927. ^ Guyenot, 1927. 



^ Schotte, 1926 a; Guyenot and Valette, 1925; Bischler, 1926. 



* Huxley, 1932. 



