S4 THE BIOLOGY OF THE FROG chap. 



and also from the attacks of aquatic insects and snails, and 

 various mechanical injuries which would otherwise affect 

 them. Bernard and Bratuschek ^ consider that the jelly also 

 serves to keep the eggs warmer than the surrounding water, 

 which in the spring, when the eggs are laid, is often very cold, 

 and frequently covered with ice. The jelly is supposed to 

 act upon the principle of a hotbed, allowing free entrance 

 to the sun's rays, but checking radiation from the egg. The 





Fig. 14. — Egg in jelly. (Alter Schultze.) 



heat waves that radiate from the eggs are longer, less refran- 

 gible, and pass through the jelly less readily than the shorter 

 waves, which are abundant in the direct heat of the sun. 

 Bernard and Bratuschek - found in experimenting upon the 

 jelly that the greater the wave length, the less heat passed 

 through in comparison with an equal amount of water under 

 the same conditions. The jelly was thus shown to have the 



1 Bernard und Bratuschek, " Der Nutzen der Schleimhiillen fiir die 

 Froscheier," Biol. Centrlb., Bd. 11, 1891. ^ Ibid. 



