CH. VIIl] FROGS, TOADS, AND NEWTS 283 



the oviducal glands deposit a layer of mucus around each 

 ovum. On reaching the water this substance swells 

 enormously by absorption of water and forms the glairy, 

 transparent, slippery covering which surrounds each egg 

 in the familiar masses of frog-spawn 1 . 



The functions of this mucous envelope are manifold; 

 in process of swelling it appears to exert a powerful 

 attractive influence upon the spermatozoa and thus aids 

 in fertilisation; it resists putrefactive changes for a very 

 long while and in this way shields the. developing tadpole 

 from bacterial attack ; it holds the eggs together, rendering 

 the mass difficult to transport in the water by increasing 

 the probability of entanglement in weeds should there be 

 any current flowing along the ditch or through the pond 

 in which spawning has occurred ; it spaces the eggs from 

 each other, allowing sufficient oxygen to be available to 

 each, and entangles in its interspaces masses of green 

 swarm -spores of Algae which give out oxygen when ex- 

 posed to sunlight and thus help to aerate the eggs; it 

 interposes an impenetrable barrier between the eggs and 

 the small carnivorous Crustacea which abound in fresh 

 water, and by its slipperiness protects the eggs from the 

 attacks of aquatic birds. I have however seen newts, 

 when very hungry, attack masses of frogs' spawn and 

 gulp down portions of it with considerable difficulty : 

 nevertheless they succeeded in so demolishing the mass 

 that none of the eggs produced tadpoles. I am inclined 

 to think that there may be some unpleasant flavour about 



1 If held up to the light the outer layer of mucus is seen to be more 

 transparent and less dense than the inner. 



