PROGRESSIVE DIFFERENTIATION WITHIN A FIELD 



PROXIMAL LIMB WINDOW IN SHELL 



105 



DISTAL 



PROXIMAL 



Fig. 52. Self-differentiation of the separated proximal and distal parts of 

 the limb. Upper left: The hindlimb cut off from the embryo and cut into proxi- 

 mal and distal fragments. Upper right: Hole made in shell and fragments of 

 limb placed near blood vessels. Lower left: Differentiation of tibia, fibula, and 

 phalanges from distal fragment. Lower right: Differentiation of femur from 

 proximal fragment. 



forms an actual outgrowth from the side of the chick embryo it is possible to 

 cut the limb into two fragments — one proximal and one distal. Each of the 

 two pieces can then be cultivated by transplanting it to the chorioallantoic 

 membrane of an older chick embryo (Fig. 52). This is done by cutting a 

 window through the shell of the egg to expose the chorioallantoic membrane 

 with its large blood vessels. The region of the membrane near the blood 

 vessels has been found to be an almost perfect site for the cultivation of 

 tissue. All that is necessary is to drop the tissue near these blood vessels; it 

 will adhere to the membrane, be nourished by the blood, and continue to 

 differentiate. 



