EFFECTS OF HEAT ON TOAD'S EGG. 23 I 



C. This optimum in increased 2-3 for eggs in later stages of 

 development. 



In another set of experiments on Ranafusca, Hertwig (4) finds 

 that the maximum temperature to which the unsegmented eggs 

 can be subjected without suffering any injury is 23-24 C., while 

 this maximum is increased to 30 C. for eggs in the late segmen- 

 tation stages. The maximum temperature for unsegmented eggs 

 of Rana escitlenta Hertwig finds to be 33 C. This is also the 

 maximum temperature I have found for unsegmented eggs of 

 the toad, although eggs in the blastula stage can endure a tem- 

 perature of 38 C. for a very short time. 



Morgan has noted that the blastula stages of Rana pahtstris 

 can endure extreme cold much better than can eggs in the 24- 

 cell stages, and he also finds that the eggs of Rana tcinporaria 

 which are laid very early in the spring, can survive the tempera- 

 of freezing water for several days. This temperature would very 

 soon kill eggs of Rana pahtstris which are deposited much later 

 than are the eggs of Rana tcinporaria. 



While the eggs of all of these species of Amtra can withstand 

 a wide range of temperature without injury, there appears to be 

 an adaptation to temperature corresponding to the different 

 periods at which the eggs are deposited. Rana fusca and Rana 

 tcinporaria lay their eggs very early in the spring when the water 

 is often at the freezing point ; and the eggs of these two species 

 can stand cold much better than can the eggs of Rana pahtstris and 

 Rana esculenta which are laid considerably later. Although the 

 eggs of Bufo lentiginosus are laid but little later than are those of 

 Rana pahtstris, they are usually deposited in shallow pools of 

 water exposed to the direct rays of the sun. They must, there- 

 fore, often be subjected to a comparatively high degree of heat 

 during the course of their development. 



BRYN MAWR COLLEGE, 



BRYN MAWR, PA., April 24, 1903. 



