l6 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



to develop an instinctive agitation of its toes. There would have to be 

 other modifications, of course, but these foam nests do not seem quite 

 so different from anything that normal species produce. 



The jelly envelopes are so lacking in nutritive properties that there 

 are few animals in the pond that will dehberately eat them. Moorhens 

 {Gallintda chloropus) occasionally eat the eggs, and mince up the whole 

 egg mass in order to reach the vitelh, leaving most of the jelly floating 

 on the water. This behaviour of the birds seems to be erratic. In some 

 years they will attack the eggs, but in others, even in the same pond, 

 with both eggs and birds present, the eggs are left entirely alone. 



It has usually been considered that the tadpoles feed on the envelopes 

 after they hatch, but this is not so. The tadpoles grow considerably 

 during the time that they remain on the envelopes, but this increase 

 in size is solely due to an increase in the water content of the animals, 

 which at this time are building up their bodies from the store of 

 concentrated nutriment provided by the parent frog. This they do by 

 forming tissues containing much more water than the yolk, and by 

 constructing organs many of which are hollow, and so take up still 

 more space. These conclusions were reached by measuring the dry 

 weight, the nitrogen and the carbohydrate of the developing eggs and 

 embryos of several species after hatching, and it was possible to show 

 that, with some minor comphcations, there was a drop in all three 

 properties until the little animals began to feed on the food particles 

 in the pond, some of which, it is true, are on the surface of the 

 envelopes (Savage, 1937, 1938). They grow as fast when they are 

 removed from the envelopes. Moreover, there is a septum closing 

 the mouth at this stage, so that the tadpole could swallow no food. 

 This septum does not break until the tadpole is about to leave the 

 neighbourhood of the envelopes. When it is also recalled how dilute 

 this jelly is, these facts are not surprising, for even the water of the 

 pond often contains as much nutriment, and an animal could get as 

 much benefit by gulping down water as by eating the envelopes. 

 Figs. 2 and 3 illustrate these experiments, and it can be seen that 

 hatching is a mere incident in the life of the tadpoles. It probably 

 occurs at a stage when the growing need for more oxygen and the 

 means for eliminating carbon dioxide and other excretory products 

 make it impossible for the young animal to remain any longer inside 

 the jelly, but the newly hatched tadpole is in other respects just an 

 embryo that has escaped from the envelope. 



