THE MOSAIC STAGE OF DIFFERENTIATION 



227 



IS no 



only roughly, and their frontiers appear to overlap. There 11 . 

 doubt that a cell which in one experiment forms part of the thigh, 

 would, in another experiment with the cut in a slightly different 



Fig. no 



Mosaic determination within the hind-Hmb rudiment of the chick. A, Com- 

 plete hind-limb developed from a chorio-allantoic graft of a whole limb-bud in 

 the stage shown at B. A graft of the distal half of the limb-bud shown at C 

 resulted in a distal half-limb (D). A still smaller distal region (E) produced only 

 a foot (F). In D and F, the sub-regions (shank, foot) are complete. (After 

 Murray, from Wells, Huxley and Wells, The Science of Life, London, 1929.) 



place, form part of the shank. The cuts must have roughly separ- 

 ated the sub-zones from one another, and each sub-zone, though 

 irreversibly determined to give rise only to its own segment, is 

 still capable of regulation to give a z^/zo/^ sub-zone. In a similar way, 



15-2 



