EMBRYOLOGY 1 85 



out from the cut stump within a period of three or four weeks. Such a 

 regenerated limb will be complete in every detail, including upper arm, 

 forearm, hand, and fingers. During this process of regeneration new skeletal 

 elements, new muscles, new blood vessels, and so on are formed. The re- 

 generated Hmb is as perfect, structurally and functionally, as the original 

 one. 



If the processes underlying the regeneration of structure, such as limb, 

 were really understood we would be a long way toward understanding 

 many of the fundamental processes of embryonic development. We know 

 that, after removal of a limb, a group of cells gradually appears at the 

 point of amputation. These cells in many respects represent a type of 

 embryonic cell. From this aggregate of cells, called a regeneration blastema, 

 is formed nearly all new structures of the regenerated Umb. But what 

 governs the growth and differentiation of the blastema cells into new 

 structures? What determines that certain cells in the aggregate will give 

 rise to skeleton, others to muscle, and so on? 



In an attempt partially to analyze the situation, one of my students. Dr. 

 Thornton, removed all of the skeletal elements from the limb stump at the 

 time of amputation. The method adopted was briefly as follows. Using a 

 young salamander, he amputated a fore limb through the middle of the 

 upper arm. Then the humerus was carefully exarticulated from its socket 

 at the shoulder joint and completely removed. Such a procedure left the 

 upper arm as a collapsed limb stump, made up of muscles, connective 

 tissue, blood vessels, and nerves, but entirely devoid of the skeleton. What 

 type of limb will regenerate from such a deficient upper arm? 



The results from these experiments showed that such animals regener- 

 ated normal limbs. As the regeneration blastema formed, it was observed 

 that some of the cells developed into a new humerus, regardless of the 

 fact that none of the old humerus was present to take part in the process. 

 Here, then, is new evidence for the assertion, that the direction in which 

 cells differentiate depends somehow on their situation and surroundings. 

 There is evidence, that under one set of circumstances certain cells may go 

 into the formation of muscle, whereas under other conditions it is possible 

 that the same type of cells will form skeleton. In an active field of limb re- 

 generation, therefore, it is clear that what happens to individual cells 

 depends on the relation of the cells to each other and to the field as a whole, 

 as w-ell as on their inherent potencies. 



Recently we have been able to secure still further information regarding 

 the manner in which cellular interaction is involved in regeneration. Pro- 

 fessor O. E. Schotte of Amherst and I have undertaken an investigation 

 of the alterations which occur in an amphibian limb rendered nerveless 

 and then amputated. 



It has long been known that the presence of nerves is essential for re- 

 generation of an amphibian limb. By devising a suitable operative tech- 



