. The elements of experimental embryology. Embryology, Experimental; Embryology. 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 de
. The elements of experimental embryology. Embryology, Experimental; Embryology. 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, t
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