226 



THE MOSAIC STAGE OF DIFFERENTIATION 



It is impossible to imagine that the cuts which were made passed 

 exactly in each case between the limits of the zones allocated to 

 thigh, shank, and foot, and it is necessary to conclude that these 





I • 



11 



V 



I 



Fig. 109 

 Mosaic determination and partial regulation within the limb-rudiment of the 

 chick. Differentiation of a small basal fragment of a very early (4-day) left hind- 

 limb bud, grafted on the chorio-allantois of another egg after 5 days. Right, 

 microphotograph of entire graft, in longitudinal section. The connexion with the 

 chorio-allantois is seen on the right: the graft has differentiated into a femur, 

 7*5 mm. long, mesenchyme, and some muscle-fibres (right bottom). Left, re- 

 construction of skeletal elements. The curve of the bone is in the same direction 

 as in a normal left femur, i , head ; 2 , shaft ; 3 , ectopic fragment of pelvis ; 4, sheath 

 of perichondral bone; 5, attachment of muscles (on far side) ; 6, patella; 7, tro- 

 chanter. Being the basal region, the graft has formed only basal structures ; there 

 has however been some intra-regional regulation, leading to the formation of a 

 femur complete at either end. (From Murray and Huxley, Joz/rw, Anat. lix, i 925 .) 



segments or constituent parts of the limb are roughly determined 

 at varying levels down its length, but that they are determined 



