No. 4-] THE DEVELOPMENT OF ANIMALS. 189 



There is a slight decrease in the rate of growth beyond the 

 optimum in the case of the tadpole of the frog. This is prob- 

 ably also the case with the toad, but our observations are not 

 sufficiently extensive to show it, for the material gave out 

 before it could be tested. 



In both the frog and the toad tadpoles the rate of increase 

 is relatively slow up*to about 20, when the rate becomes sud- 

 denly much more rapid. This is very marked in the tables ; for 

 instance, the elongation of the tail in the frog tadpole is 1 1.16 

 units at 20 in 24 hours, at 25 it is 22.75, or more than twice 

 the amount at 20; in the toad tadpole the figures are 10. 16 

 and 28 at approximately the same temperatures, an even more . 

 striking increase. The curves show this too; see especially 

 the curve for growth in length of the tail of the toad tadpole. 

 Why there should be this sudden change in rate between 20 

 and 25 is difficult to determine. 



b. Snbminimal and Supra/maximal Temperatures. As was 

 to be expected, we found that very low temperatures - - that is, 

 below 3 in the case of the frog and 6 in the case of the 

 toad - - entirely inhibited growth. But we obtained one result 

 which was entirely unexpected : at about 2 C. there was an 

 actual shortening in the tadpoles of the frog in twenty-four 

 hours ; this decrease in length was very slight, but sufficiently 

 well marked to be plainly discernible, varying from .5 to 2 

 units. It was most plainly shown in the youngest tadpoles. 

 We have come to the conclusion that the decrease is due to a 

 diminution in the turgor of the cells, caused by diminished 

 endosmosis dependent either on the low temperature directly, 

 or on the non-production of the active endosmotic substance 

 within the cell, or on both factors combined. In this connection 

 the observation of Davenport ('97) that "the immense increment 

 in weight which accompanies the outlining of the form of the 

 larva (tadpoles of frog and toad) and its organs is due almost 

 solely to absorbed water" is of importance. 



Temperatures above the maximum for normal growth re- 

 sulted as a rule very quickly in death of the tadpoles. There 

 was, however, a marked tendency in the case of those that 

 survived a sufficiently long time for the tail to grow out at an 



