6 



THE AMERICAN NATURALIST 



[Vol. XLII 



possible that the presence of such tissue might excite 

 the leg to regenerate, but this did not prove to be the 

 case. Possibly, here also, the difference in species 

 spoiled the result, but this seems unlikely, since it has 

 been shown that young tadpoles of different species may 

 be readily united and form permanent unions. 



It is well known that the skin of the frog has excellent 

 powers of regeneration, yet when the leg is cut off it 

 generally ceases further growth after the cut surface has 

 healed over. It occurred to me that the pressure of the 

 skin over the cut end might in itself prevent the further 

 growth of the internal organs. To examine this possi- 

 bility I cut off the fore-leg turned back the skin, cut off 

 a piece of the stump, and then sewed up the end of the 

 pocket. There was thus left a free space between the 

 cut end of the stump and the end of the skin. Neverthe- 

 less regeneration of the limb was not hastened. In other 

 cases agar, or the coagulated white of the hen's egg, was 

 inserted into the pocket, on the supposition that as they 

 were absorbed the regeneration might take place but 

 nothing of the sort occurred. These experiments were 

 undertaken before I became aware of the fact that the 

 fore-leg of an adult frog may actually regenerate, but 

 only after a long time and imperfectly, so that the experi- 

 ments would be expected at most to facilitate or hasten 

 the regenerative process which however they did not 

 seem to do. In the winter of 1904-05 one frog that lived 

 for several months, floating in the water, regener- 

 ated a long outgrowth from the cut end of a fore-leg. 

 The new part had one or two rudimentary finger-like 

 outgrowths, but although the outgrowth was long it was 

 very imperfect as a limb. This frog had had its leg cut 

 off twice, the second time two months after the first 

 operation. In some preparations of Mr. Goldfarb I had 

 seen the great thickening of the periosteum that takes 

 place after several weeks, and this suggested, that if the 

 leg were cut off anew at this time, regeneration niMit 

 take place. I have also obtained more recently two or 



