1 06 EMBRYOLOGY 



The results of the experiment just outlined are as follows: The proximal 

 region forms an elongated mass of tissue and within this mass of tissue 

 will be found a structure — a bone which appears to be an almost perfect 

 femur. The rest of the tissue develops into muscle around this femur. The 

 distal half of the limb bud forms a mass of tissue shaped into the general 

 structure of the distal parts of the nind-limb and containing a tibia, a very 

 slender fibula, the fused tarsals and metatarsals, and the phalanges forming 

 the digits. In the original transplants of proximal and distal halves there are, 

 needless to say, no indications whatsoever of any of these structures. The 

 transplant consists merely of masses of undifferentiated mesoderm which 

 have grown out from the original limb disk. But at this limb bud stage a vast 

 amount of detail will differentiate from the parts of the limb bud. 



As development of the limb proceeds, more and more detail is deter- 

 mined in the limb. In a later stage, for example, the limb grows out and the 

 first digit appears as a separate outgrowth from the rest of the digits. If the 

 tip of the limb is cut off without including the first digit, this distal portion 

 will develop into a foot which lacks the first digit. The phalanges of the 

 second, third, fourth, and fifth digits will develop, but the first will be lack- 

 ing. Therefore a minor structure such as the first digit is now determined and 

 the rest of the tissue is not able to form one. 



Once again there is an obvious similarity between this phenomenon of 

 progressive determination of parts of an organ and the changes that occur in 

 the whole egg. Before gastrulation we found, by a variety of demonstrations, 

 that the parts of the egg were not determined. After gastrulation the major 

 parts are determined and cannot be interchanged. In an early stage of the 

 limb disk the parts of the limb are not determined. But in a somewhat later 

 stage the structures within the limb bud become determined in some detail. 

 The presumptive femur will self -differentiate. 



Emphasis has been placed on the determination of the limb in the later 

 stages of its development. We cannot, however, think of this determination 

 as being final or irreversible, because even when the limb has completely 

 differentiated in full detail in the adult, parts of the limb may still regenerate 

 new structures. This problem will be considered later. All that needs to be 

 pointed out now is that the fully formed structure may still have some extra 

 potencies left in it and that in some animals at least these potencies can be 

 expressed by the regeneration of lost parts when the limb is injured or cut 

 and parts are removed. 



