284 FKOGS, TOADS, AND NEWTS [CH. VIII 



the mucus, for I have seen toads at the spawning season 

 killed in large numbers by crows who attacked them 

 before spawning had actually been completed: the birds 

 picked the toads to pieces, devouring all the muscles, 

 liver and intestines but in every case they left the skin, 

 which is known to be extremely acrid, the oviducts and 

 such ova as were coated with mucus of the ovaries them- 

 selves not a trace could be found. Some of the spawn 

 had already issued in its characteristic ropes, and was still 

 continuous with other portions lying within the oviducts. 



In its normal condition frogs' spawn floats in the 

 water. It is probably buoyed up to some extent by 

 bubbles of oxygen arising from submerged water plants. 

 If removed from the water and carried in a small vessel 

 it will frequently sink when placed in an aquarium : this 

 however does not seriously interfere with the normal 

 development of the young. The natural position at the 

 surface of the water is however of importance. It places 

 the eggs in the warmth of the sun, and it is noteworthy 

 that in ditches the spawn is usually deposited against 

 the more sunny bank. It is possible that to a small 

 extent the convex surface of the mucous envelopes con- 

 centrates the rays of the sun upon the eggs within. Each 

 egg is black in its upper half and, at first, white beneath ; 

 hence the surface exposed to the sun has the greater 

 power of absorbing heat. 



The precise rate of development of the fertilised egg 

 depends to some extent on the temperature, so that 

 the periods mentioned in the following paragraphs must 

 be taken as averages. In about four days the black 



