100 



EMBRYOLOGY 



WOUND HEALS 



TRANSPLANTED 

 ANOTHER LIMB FORMS LIMB DISK 

 FROM TRANSPLANT 



Fig. 48. Two limbs from a sin- 

 gle limb region. A disk of tissue 

 is removed from one embryo and 

 transplanted to another. One limb 

 develops from the transplanted limb 

 disk and a second limb forms from 

 tissue which fills in the wound. 



by transplant to another embryo and another from the wound in the host. 

 This experiment shows that two — and other experiments show that even 

 three — forelimbs can be obtained from one presumptive forelimb. Still other 

 experiments demonstrate that two forelimb disks may fuse together and may 

 mesh perfectly to form one single limb. To obtain this result the polarities of 

 the two limb disks must coincide. 



The above experiments show a rather striking similarity to the experi- 

 ments carried out on the unfertilized egg. It will be remembered that we 

 could get two embryos from one egg by cutting it in two. We also obtained 

 a single embryo by fusing two eggs together. The same relationships hold 

 for the limb disk. Thus the limb region can be considered to be the germ for 

 the limb, just as the egg can be considered to be the germ for the entire 

 individual (Fig. 49). The same type of labile organization is present in both 

 these developing systems. In the sea urchin egg (Chap. 3) the skeleton, the 

 gut, and the ciliated ectoderm are not determined in any detail. It is obvious 

 in the case of the limb disk that the digits, the radius, ulna, humerus, and 

 the muscles associated with them are not determined in any detail. The cells 



