WITHIN THE EGG 185 



sun, or to the warmth of decomposing vegetable matter, as 

 in many mound-birds. And the cuckoo is notorious for 

 having found out the device of being maternally virtuous 

 by proxy. 



An important point, of course, is that the embryo is very 

 imperfectly warm-blooded, and that the development will 

 slow or even stop if there be not a requisite warmth, and that 

 even the fledglings are very imperfectly warm-blooded and 

 may die if left for some hours exposed. Young birds are 

 very imperfectly warm-blooded in those cases where brooding 

 is habitual, and it is interesting to notice that old-fashioned 

 primitive birds, such as the Emeu (like old-fashioned primi- 

 tive egg-laying mammals), are also, as adults, imperfectly 

 warm-blooded. 



Of great interest is the epoch after two to six weeks 

 when the embryo, well-formed, and it may be with a suit of 

 feathers, has used up its legacy of yolk, and must emerge. It 

 thrustsits bill at thebroad end of the eggintothe air-chamber, 

 which has been gradually growing larger ; it gets its lungs 

 inflated for the first time ; it becomes suddenly more vigorous 

 and writhes in its birth-robes ; it pecks with its egg-tooth at 

 the shell, makes a crack, a hole, a rent and sinks squirming, 

 like a half-made thing, on the floor of the nest, or steps boldly 

 out into the world of action and freedom. 



