FIELDS AND GRADIENTS 299 



Urodele limb-buds. If a limb-disc is removed and grafted on to 

 the flank of the same animal, it will develop into an independent 

 limb if sufficiently far from its original position. But if the site of 

 grafting is within three segments of its original position, it is within 

 the sphere of dominance of the limb developing from the portion 

 of limb-disc left in situ and becomes resorbed.^ Experiments on 

 Anura have had similar results.- 



If an entire limb-disc be grafted, it often develops into two or 

 three limbs. In this case the operation has upset the normal gradi- 

 ent system, and permitted supernumerary centres of activity to 

 develop. This fact is of great interest in its bearing upon dichoto- 

 mous growth ; for it shows that a field which normally gives rise to 

 a single set of structures can under slightly altered conditions be 

 made to give rise to two.^ 



Similar agencies are at work in a Hydroid or a Planarian. The 

 various regions of the body, though each capable of producing a 

 new apical region, are all held in check by the existing head or 

 hydranth. When, however, growth has removed them to a sufficient 

 distance, the inhibition can no longer act on them, and they do 

 develop into apical regions. 



The double multiple forms are of great interest. Two-headed 

 Planarians can be produced by splitting the anterior end and pre- 

 venting the two halves from reuniting; and types with doubled 

 apical region can be produced by similar means in sea-anemones.'* 

 In cases of normal dichotomy of branching organisms, the duplica- 

 tion of the axis and main structures is brought about by growth ; 

 however, what initiates the division of the growing-point in these 

 forms is not yet known. 



In fragments of hydroid stems, biaxial hydranths are formed when 

 conditions at the two ends are such as to produce two positive 

 gradients of sufficient intensity to initiate the formation of an 

 apical region. Neither region has a sufficient advantage to inhibit 

 the development of the other. If either cut surface is handicapped 

 by being enclosed in paraffin or stuck in the sand, only the other 

 end produces a new hydranth. In Tiibularia the stem is enclosed in 



1 Detwiler, 1918. " Hellmich, 1930. 



^ The symmetry relations of the supernumerary limbs are of much interest: 

 this problem is considered on p. 224. 



^ Child, 1924, p. 161. See also below, p. 327. 



