IO MORGAN. [VOL. I. 



the effect of the ice-cold water. If segmenting eggs or the 

 early gastrula stages be put on ice they are killed after several 

 days, although the latter stages withstand the cold longer than 

 the former. The young tadpoles do not seem to be in the least 

 injured, and may even slowly continue to develop, but at so 

 slow a rate that after three weeks the tail had grown only a 

 very little. Since in this locality the eggs of R. palustris can 

 be obtained in great abundance for a period of at least two 

 weeks, I have had plenty of material of both species. 



The tadpoles were operated upon at the time when they had 

 reached the age shown in Harrison's Fig. 2. They were still 

 in the jelly membranes. Muscular movements of the body had 

 scarcely begun at this time. The tadpoles were held in place 

 by small pieces of aluminium wire. Silver wire used by Born 

 and by Harrison would probably be better, since it is heavier. 



The young tadpole of R. sylvatica is very black, the color 

 being due to the deeply pigmented ectoderm and to some 

 extent to pigment in the mesoderm. The young tadpole of R. 

 palustris is much lighter in color. The ectoderm contains a 

 yellowish pigment, and the pigment cells of the mesoderm are 

 lighter in color than those of the other species. After grafting 

 together parts of these two species, the difference in color of 

 the two parts is so marked that it can be easily seen with the 

 naked eye. Under the microscope one can tell readily whether 

 an individual cell in the ectoderm belongs to the one or to the 

 other species. In later stages, when the ectoderm has become 

 clearer, the two kinds of cells can no longer be distinguished 

 without a microscope. The core of the tail of R. sylvatica is 

 much darker than that of R. palustris, and the line of union of 

 the two can be seen with the unaided eye. 



As the tadpole grows larger, it will be found that the ectoderm 

 of the smaller component grows less rapidly than the rest of the 

 tail, and as a result the ectoderm of the larger component extends 

 over the base of the grafted tail, as Harrison has stated (Figs. 2 

 and 3). The tadpoles were allowed to grow for about ten days, or 

 somewhat longer, 1 and then the tail was cut off in various ways. 



1 It would have been better to have cut the tail off sooner, since the difference 

 in the ectoderm of the two species is less marked in later stages. 



