224 



THE MOSAIC STAGE OF DIFFERENTIATION 



that the preaxial border of the Hmb (marked by the first digit and 

 radius in the fully developed limb) will arise from the anterior 

 portion of the limb-disc. Although the limb-field is regionally still 

 undetermined, it is polarised along an antero-posterior axis from 

 the first moment at which its existence can be detected: this is 

 proved by grafting limb-discs in abnormal orientations^ (figs- 107, 

 108, 173). 



A B 



Fig. 108 

 Limb determination in Amblystoma. A, Middle gastrula stage showing pre- 

 sumptive limb area which was removed and grafted with reversed orientation into 

 a neurula (B), where it developed into a limb with left asymmetry, though on 

 the right side, like that shown in fig. 107. This proves not only that the limb is 

 determined at the middle gastrula stage, but also that its antero-posterior axis is 

 already determined. (From Detwiler, Jo^rw. Exp. Zool. lxiv, 1933, figs. 2, 3.) 



The hind-limb field in Amblystoma extends from the level of the 

 sixteenth to the eighteenth trunk segments inclusive, and shows 

 properties similar to those of the fore-limb field. ^ Its determina- 

 tion and differentiation takes place later than that of the fore-limb 

 field. 



^ A common occurrence when portions of limb-discs are grafted is the fact 

 that they give rise to reduplications, i.e. monstrous double or even treble limbs 

 are formed connected with one another at some point along their length. This 

 in itself is merely another example of the fact that the limb-area is as yet only a 

 field and not a regionally determined rudiment. But these reduplications are of 

 interest from another point of view, for the reduplicated member is as a rule a 

 mirror image of the original member. They therefore supply an illustration of 

 Bateson's rule, which may be forrriulated as follows: (i) the long axes of re- 

 duplicated structures lie in the same plane ; (2) two reduplicated limbs are mirror 

 images of one another about a plane which bisects the angle between the long 

 axes of the members, and which is at right angles to the plane of these axes. 

 The detailed explanation of reduplication and mirror-imaging has given rise 

 to considerable controversy. See Harrison, 1921 a; Przibram, 1924; Mangold, 



1929 A. 



Stultz, 193 1. 



