98 



EMBRYOLOGY 



In the egg the first thing noted was a polarity, involving a difference between 

 the animal and vegetal poles. In the case of the amphibian limb we also find 

 a polarity. And this is demonstrated by the following type of experiment 

 (Fig. 47). 



In a young embryo the limb bud can be localized by its relationship to the 

 mesodermal somites, and when a certain area of the lateral mesoderm about 

 three somites in diameter is transplanted it will differentiate into a perfect 

 limb. We can indicate the polarity of the limb disk by the letters A and P, 

 for anterior and posterior regions, and follow the limb polarity in transplants. 

 In normal development the limb grows out from the disk in a posterior direc- 

 tion and the digits take on shape. Notice that the long digit is ventral and 

 there are three small digits on the dorsal side. Thus we find an anterior- 



NORMAL ORIENTATION 



LIMB DISK TURNED 180° 



OBSERVED RESULT 



EXPECTED RESULT IF 

 POLARITY DETERMINED 



Fig. 47. The polarity of the limb disk. Upper figure shows an amphibian 

 embryo, the approximate size and location of the limb disk, and the direction 

 of outgrowth of the limb, A for anterior, P for posterior. Lower left: The 

 observed result when the limb disk is cut out and rotated through 180°. Lower 

 tight: The expected results if the polarity were completely fixed. 



