Chapter VIII 



FIELDS AND GRADIENTS 



§1 

 If a simple animal such as a Planarian is cut transversely into two 

 pieces, normally the front piece will form a tail at its hind end, and 

 the hind piece will form a head at its front end. But if the trans- 

 verse cut had been made a short distance farther back in the body, 

 those cells which in the previous experiment belonged to the hind 

 piece and proliferated to form a head, will now belong to the front 

 piece and will proliferate to form a tail. Therefore the determina- 

 tion of the quality of the structure which is formed cannot be 

 based on any localisation of specifically different materials or 

 substances, for, if so, it would be impossible to understand how 

 either a head or a tail can be formed from identically the same 

 tissues. How, then, is the quality of the structure which will be 

 formed determined?^ 



A situation in some respects comparable with that just described 

 occurs in the regeneration of the limbs of newts. An amputated 

 limb gives rise to a regeneration-bud, from which an arm or a leg, 

 as the case may be, is eventually formed. These structures can be 

 easily distinguished by the number of digits and other criteria. 

 But at the outset of this process of regeneration there is no 

 qualitative determination of arm-forming as opposed to leg- 

 forming material in the regeneration-bud, for an arm regeneration- 

 bud can be grafted on to the stump of an amputated leg, where it 

 will develop into a leg, provided that the operation is performed 

 soon enough after the amputation of the arm and the formation of 

 the arm regeneration-bud. The converse experiment of grafting a 

 leg regeneration-bud on to the stump of an arm leads to the for- 

 mation of an arm under the same conditions.^ 



The tissue regenerated by an arm or a leg is at the outset not even 

 determined to produce a limb. The early regeneration-bud of a 



^ See also J. Loeb, 1912. - Milojewic, 1924. 



