THE MOSAIC STAGE OF DIFFERENTIATION 225 



By this stage, too, the growth-coefficient of the limb relative to 

 the body has also been determined, as is shown by heteroplastic 

 experiments in grafting limbs between slow-growing and fast- 

 growing species of Amhlystoma. Limbs of the fast-growing species 

 on the body of the slow-growing one become disproportionately 

 large, and vice versa} (See fig. 203, p. 421.) 



It is only at later stages that the unitary limb region, which forms 

 one of the major pieces in the mosaic of the whole organism, itself 

 becomes converted into a mosaic of invisibly determined sub- 

 regions. The precise time of onset of this stage varies in different 

 forms. In Ambly stoma punctatum it appears to be reached when the 

 visible limb-bud has attained a markedly conical form. The organ- 

 ism is then a larva with well-developed external gills and tail. In 

 Triton taeniatus, on the other hand, it appears to set in relatively 

 earlier, in the tail-bud stage. ^ In Triton, the limb also develops 

 relatively earliei than in Ambly stoma punctatum, but there is no 

 correlation between time of determination and time of develop- 

 ment, for in Amhlystoma tigrinum^ determination sets in earlier 

 but development does not occur until later than in Amhlystoma 

 punctatum. 



When the stage of regional determination of subregions within 

 the field has been reached, division of the rudiment will no longer 

 result in the formation of two limbs by regulation, but each portion 

 will give rise to a partial structure. Progressive chemo-diflFerentia- 

 tion has taken place, and within the main limb-field a secondary 

 mosaic has been formed, each region of which, however, is still 

 indefinitely determined and therefore capable of regulation. 



The analysis of these late stages has been undertaken in the limbs 

 of the embryo chick. By grafting portions of the limb-bud of a 

 4-day chick on to the chorio-allantois of another Qgg, it is found that 

 if the limb-bud is divided into pieces by cuts at right angles to its 

 future long axis, the proximal piece differentiates into a perfect 

 femur, the next piece into a perfect tibia and fibula, and the distal 

 piece into a perfect foot. It is important to note that even the 

 structure of the joints appears to be predetermined in almost all its 

 details.* (See figs. 109, 11 1.) 



Harrison, 1924 a; Huxley. 1932. ^ Brandt, 1924. 



Ruud, 1926. * Murray, 1926. 



15 



