222 



THE MOSAIC STAGE OF DIFFERENTIATION 



principle, we may turn to the phenomena presented by some of the 

 presumptive organ-regions in Amphibia, beginning with the Hmbs. 



C C 



Fig. 1 06 



Diagram of the fate of the four quadrants of the fore-Hmb field in Anihly stoma. 

 The Hmb-disc is shown at A in each case, with one quadrant stippled. The dotted 

 lines intersect at the point of maximum limb-forming potency. At C in each case 

 is shown the young limb, viewed dorsally (above) and laterally (below), showing 

 the portions derived from the stippled quadrant. (From Swett, Journ. Exp. 

 Zool. xxxvn, 1923.) 



In Amblystoma, the presumptive fore-Hmb area or field occupies 

 a discoid zone on the side of the body, extending from the anterior 

 margin of the third trunk segment to the middle of the sixth. The 



