l8o LILLIE AND KNOWLTON. [VOL. I. 



extents of development in the same time at different tempera- 

 tures in terms of each other : if for instance we discover that 

 at a certain temperature the egg of the frog develops in twenty- 

 four hours to the blastula and at another temperature to a later 

 stage of gastrulation, we cannot express the effect of the 

 increase in temperature quantitatively ; but if, on the other 

 hand, a fixed extent of development takes two days at one tem- 

 perature and one day at a higher temperature, the effect of the 

 increase in temperature is given quantitatively. However, in the 

 case of mere growth in bulk or length, the extents at different 

 temperatures within the same period of time can be directly 

 compared. 



In our study of the effects of temperature on development 

 we have included (i) the regeneration of Planaria torva; (2) 

 the cleavage and gastrulation of Amblystoma and the frog ; 

 and (3) the rate of growth of the tail of tadpoles of the frog 

 and toad. The observations made have been much more numer- 

 ous and have taken up much more time than the tables would 

 lead one to suspect ; this is due to the necessity of repeating 

 observations to secure averages so as to eliminate the factor 

 of individual variation, and to the great mortality at high tem- 

 peratures. The temperatures below 22 C. are in all cases 

 averages, owing to the impossibility of securing perfectly uni- 

 form low temperatures during the time occupied by the experi- 

 ments. The average temperature was calculated from daily 

 readings in the longer experiments, and from more frequent 

 readings in the shorter ones; the variation from the average 

 was rarely more than one degree either way. 



i. Planaria torva. 



a. Normal Temperature Range. --The animals were cut 

 transversely through the middle of the body, thus dividing the 

 pharynx. The time of regeneration of a complete head on the 

 posterior half at different temperatures was then measured. 



From the table it is seen that the lowest average tempera- 

 ture at which regeneration took place was 3 C. : of six speci- 

 mens at this temperature only one regenerated at all, and in 



