EFFECTS OF HEAT ON TOADS EGG. 221 



normally, and sections made of later embryos showed them to be 

 no different from the embryos of the control set. 



Experiment 5. A bunch of about seventy-five unsegmented 

 eggs was put into water heated to a temperature of 3435 C. on 

 April 1 6. Part of the eggs were removed at the end of half an 

 hour and a few of them at once began to segment. None of the 

 cleavage planes, with the exception of the first, came in normally, 

 and in no case did any of them cut through the entire egg. 

 Part of a section of one of these eggs is shown in Fig. I. All 

 of the cleavage planes are seen to be parallel and to extend but 

 a short distance through the upper hemisphere of the egg. De- 

 velopment did not progress beyond this stage in any case, and 

 the majority of the eggs never segmented although they appeared 

 to be living several hours after they were brought into room 

 temperature. 



Some of the eggs of the above lot remained at the temperature 

 of 34-35 C. for one hour. When put into cooler water and ex- 

 amined, a slight depression was found in the center of the upper 

 hemisphere of a few of the eggs as if the first cleavage plane was 

 about to appear in its normal position. This appearance, how- 

 ever, proved to be only a wrinkling of the surface as none of the 

 eggs, when sectioned, showed any true cleavage planes. 



The above experiments show that the unsegmented eggs of 

 the toad can withstand a temperature of 32-33 C. for one-half 

 of an hour and develop normally, while an exposure to this 

 temperature for a longer period is very injurious and only a small 

 per cent, of the eggs produced normal tadpoles. Exposure to a 

 temperature of 34, even for a short time, injures the eggs be- 

 yond the possibility of a recovery. The maximum temperature 

 that the unsegmented egg can endure without injury is, there- 

 fore, 33 C. The optimum temperature, a term defined by Hert- 

 wig (3) as, " Die Temperatur bei welcher sich der Entwicklungs- 

 process bei alien Eiren mit der grossten Beschleunigung ohne 

 eine auffallige Storung und Abweichung von der Norm vollzieht," 

 for this egg is probably not far from 28 C., judging from the re- 

 sults obtained in experiments I and 2. In all cases in which the 

 heat did not kill the eggs, development was accelerated at first, 

 apparently with no injurious effects on the egg. In later stages, 



