THE MOSAIC STAGE OF DIFFERENTIATION 



223 



limb potencies are restricted to the mesoderm of this region, 

 the ectoderm not being predetermined in any way.^ The first 

 important point to notice is that within this limb-disc there is 

 no definite spot or area which is necessarily destined to form a limb 

 in normal development : all regions of the field have the power of 

 forming a limb, and the extent of the field is greater than the region 

 which actually does form the limb in normal development.^ The 

 Hmb-field is already determined at the middle gastrula stage.^ 



The limb-forming potencies are highest in a subcentral region 

 of the field, situated near to its anterior and dorsal margins, and 

 grade away from this.^ A normal limb can be formed from half the 



Fig. 107 

 Polarisation of the limb-field. Axolotl in which a limb-disc from the right side of 

 the body has been grafted on to the same side, a little way behind the normal 

 limb, the correct side out but with the antero-posterior and dorso- ventral axes 

 reversed. It has developed into a limb (TR) with correct dorso- ventral relations, 

 but with the preaxial border facing the tail of the larva ; consequently it possesses 

 left-handed asymmetry. (From Harrison, Jowrw. Exp. Zool. xxxii, 1921.) 



limb-field, either from what is left in situ after removal of half, or 

 from a half grafted elsewhere : a single field may therefore give rise 

 to two perfect limbs. Conversely, a single perfect limb can be 

 formed from two half-rudiments grafted together (provided only 

 that their antero-posterior axes are coincident, see below, p. 224 

 and also pp. 357, 418). From a very early stage, therefore, the 

 limb-field is irreversibly determined as a whole to give rise to 

 limb-tissue, but there is as yet no regional determination within 

 the field, of the constituent parts of the future limb. In addition 

 to the fact that a limb will arise somewhere within the limits of 

 the field, there is only one additional determination, and that is 



^ Harrison, 1918; Detwiler, 1918. 

 3 Swett, 1923. 



Detwiler, 1929 a; 1933 a. 



