902 GENERAL METABOLISM [pt. iii 



water-content of the envelope is not attained until after the egg 

 has been deposited in water, but interesting and suggestive modifica- 

 tions are found in the eggs of those amphibia which deposit their 

 eggs on land. In Phyllomedusa and in Rhacophorus the protective 

 function of the mucoid envelope is to a large extent replaced by 

 other devices, and it is difficult to resist the conclusion that the 

 envelopes themselves are largely devoted to the provision of water. 

 In Phyllomedusa hypochondrialis the eggs are deposited in the folds of 

 leaves. The mucilaginous egg-capsules rapidly Hquefy after ovi- 

 position and provide a fluid medium in which the eggs develop. 

 Agar observed that a certain percentage of capsules contain no eggs, 

 and this suggests that the function of these membranes is to augment 

 the amount of water available for the larva. The essential point is 

 that the whole of the water necessary for development is provided 

 by the walls of the maternal oviduct. Similarly the eggs of Rhaco- 

 phorus schlegelii are laid in a subterranean burrow. Having formed 

 this burrow, the female secretes into it a mucilage which with the 

 aid of her feet is rapidly worked into a froth. Into this froth the eggs 

 are laid and as development proceeds the froth is gradually liquefied. 

 Here again all the water for development is derived from the female 

 organs. From these types it is not difficult to derive either the egg 

 of a reptile with its solid albumen phase or the egg of a bird with its 

 fluid albumen which has entirely lost the power of protecting the 

 embryo against predatory foes. It is interesting that far from re- 

 quiring a supply of water from external sources, the eggs of birds 

 fail to develop unless a certain amount of water is lost by evaporation 

 during incubation, as Chattock has shown. And a suggestive experi- 

 ment of Weldon's, who incubated eggs in such a way as to replace 

 the amount of water normally lost by evaporation, indicates that 

 the proper formation of the amnion is dependent on loss of water 

 by evaporation. 



"Since the mammals are derived from the oviparous reptiles it is 

 of interest to consider how the small eutherian egg can be derived 

 from that of the latter group without any break in the physiological 

 functions of the organs concerned. A conceivable line of origin is 

 suggested by the eggs of monotremes. These have no true albumen 

 layer and the yolk ovum lies close under the shell. As it leaves the 

 ovary, the egg is about 2 mm. in diameter, but during its passage 

 down the oviduct its bulk is enormously increased so that the yolk 

 is about 14 mm. in diameter before the shell is deposited (Caldwell). 



