REGENERATION 443 



A careful study of the histological changes was included in this 

 investigation (Fig. 16). During normal regeneration following amputa- 

 tion in the three-digit stage, for example, healing occurred by an extension 

 of the epidermis without cell division. The next important change was a 

 vacuolation of the cartilage at the cut end of the humerus, which Butler 

 regarded as primarily a result of the mechanical injury at time of amputa- 

 tion. The changes in this region of the cartilage eventuated in what he 

 regarded as a dedifferentiation of the cartilage cells into mesenchyme 

 cells rather than a degeneration related to phagocytosis, as described by 

 certain investigators. While these changes were taking place in the 

 cartilage during the first 6 days after amputation, other mesenchyme 

 cells accumulated between the epidermis and the end of the humerus; and 

 these cells in conjunction with those arising by dedifferentiation of the 

 cartilage apparently gave rise to the blastema from which all the new 

 parts of the regenerating limb except epidermis and nerves were formed 

 in the manner described by various investigators. Mitotic figures were 

 frequently observed in the limb during normal regeneration. 



In the nonregenerating limbs of X-rayed individuals the histological 

 changes differed profoundly from these normal processes, except that 

 the epidermal healing proceeded normally. With material from an 

 experiment in which the larvae were exposed for 5 min. daily, the changes 

 during the first 2 or 3 days following amputation were not obviously 

 different from those in the normal regeneration (Fig. 16). On about the 

 fourth day there was observed the beginning of an extensive alteration 

 of the cartilage of the humerus. At the outset this was not unlike the 

 dedifferentiation described for the control specimens but was more 

 extensive and finally resulted in complete disappearance of the humerus 

 in 13 to 16 days, and then of the scapula some 18 days after the amputa- 

 tion. Butler did not regard this as a degeneration process in the ordinary 

 sense, because there were very few evidences of degenerating cartilage 

 cells and no accumulation of blood cells about the end of the humerus as 

 would have been expected if phagocytosis had been in progress. As the 

 dissolution proceeded, released cartilage cells became indistinguishably 

 mingled with the cells of what he called the "pseudoblastema" in a 

 manner comparable with the mingling of mesenchyme cells in this region 

 during the normal regeneration. Thus, about 20 days after limb amputa- 

 tion the irradiated larva possessed a nonregenerating limb stump totally 

 devoid of skeleton. 



This dissolution of the cartilage occurred in spite of the fact that the 

 limb of the opposite side, which was not amputated, developed from the 

 two-digit to the three-digit stage during the same period and in the 

 same specimens. In short, cartilage passed through stages of dissolution 

 on one side of the body while cartilage was developing on the other. It 

 was noted, however, that no fourth digit appeared on the unamputated 



