156 



EMBRYOLOGY 



segments 6, 7, and 8 were put in place of 3, 4, and 5. Segments 6, 7, and 8 

 under these conditions innervate the forelimb. But now the function of the 

 forelimb, even though it is innervated by foreign nerves, is complete. There- 

 fore 6, 7 and 8 are able to substitute for 3, 4, and 5. This result means that 

 6, 7, and 8 are plastic; they are labile enough to be changed by being placed 

 in this new position. And we assume, then, that there is some kind of induc- 

 tion, that chemical factors in the surroundings change 6, 7, and 8 into 3, 4, 

 and 5. There is still some question regarding the localization of these factors 

 in the surrounding tissues. 



Common innervation of two limbs 



The principle of labile organization holds true for the third, fourth, and 

 fifth nerves with respect to their limb function. This is perhaps best illustrated 

 by the transplantation of an extra limb bud to a site very close to the normal 

 limb bud. Under these conditions two limbs develop (Fig. 92). The normal 



TRANSPLANTED LIMB DISK HOST LIMB DISK 



5TM NERVE SUPPLIES BOTH LIMBS 



Fig. 92. Two limbs having a common innervation by the fifth spinal nerve. 

 A presumptive limb is transplanted close to the normal limb region. Two 

 limbs develop in close proximity. Each limb receives nerve fibers from the 

 fifth nerve. The result is that both limbs respond to stimulation at the same 

 time. This is homologous function. 



