No. 3.] 



THE REGENERATION OF TISSUE. 



I have found a much safer criterion than before for distin- 

 guishing the inner tissues of the two species of tadpoles used 

 in these experiments. In my former experiments I used the 

 differences in color of the pigment cells. I find that this can- 

 not be relied upon under all circumstances. But the muscle 

 tissue of the tail of Rana palustris is, especially in the early 

 stages, golden-yellow, while in Rana sylvatica the same cells 



A (April 24). 



B (April 25). 



are slaty-white. The two kinds of cells can be easily distin- 

 guished by means of this color difference. Other experiments 

 have shown also that this difference in color is transmitted to 

 the regenerating tissues in the new tail, so that it can be relied 

 upon in the grafting experiments. 



In the first series of experiments the tails were grafted as 

 described in my former paper. After eighteen to twenty-four 

 hours, as a rule, the tail was cut off, as shown by the line a-a in 

 Fig. i. Before grafting it was found more convenient to cut 



