LABILE ORGANIZATION WITH THE LIMB DISK 99 



posterior polarity (AP) marked by the direction of the outgrowth of the limb, 

 and a dorsal-ventral polarity recognizable by the position of the long digit. 

 Are these polarities fixed ? 



Modification of polarity 



To test this possibility we can rotate the limb disk through 180° so that 

 the posterior part of the limb bud is now anteriorly located, and the anterior 

 part is posteriorly located. After this rotation of the limb bud the limb grows 

 out in an anterior direction. The limb points anteriorly instead of posteriorly. 

 That is, the anterior-posterior polarity is determined in the limb disk. But 

 the dorsal-ventral polarity is not determined, because this limb disk is rotated 

 through 180°, and if all the parts in that limb were fixed, the expected result 

 would be that the limb would grow out as in Figure 47 with the large digit 

 dorsal. This can easily be demonstrated by rotating the formed limb through 

 180°; then it will be noted that the large digit is dorsally located. But the 

 actual result of rotation of the limb disk shows that the large digit is ventral. 

 It must therefore have changed its position. The large digit should have been 

 dorsal, were it just a matter of rotating a determined, fixed structure. Thus, 

 from this simple experiment it becomes clear that the anterior-posterior 

 polarity is determined; the dorsal-ventral polarity is not. There is obviously 

 a certain amount of labile organization in this limb region. Cells which 

 normally would have differentiated into the tissues of the large digit have 

 changed their course of development and have formed the small digit. 



Labile organization within the limb disk 



Let us see just how labile the limb region is. If we apply the usual test for 

 self -differentiation — that is, if we cut out a part of the limb region and trans- 

 plant it into the flank region of another embryo — a perfect forelimb will 

 differentiate (Fig. 48). If nervous connections are made between the spinal 

 cord and this limb, it will function. The significant thing about the experi- 

 ment is this: The donor from which this limb was removed will also develop 

 a perfect, functional forelimb. This forelimb differentiates from cells lining 

 the wound left after the transplant was made. These cells close in from all 

 sides and the wound heals; and from this tissue a perfect forelimb develops. 

 Thus two forelimbs are obtained from a single forelimb region: one derived 



