172 



Wild Birds 



freely in its liquid envelope, so that whenever the entire egg 

 is rolled, the yolk, becoming top-heavy, quickly adjusts itself, 

 always keeping right side up. 



The embryo, which at first rests upon the huge mass of yolk, 

 lies across the long axis of the egg, but later it gradually shifts 

 to a lengthwise position, until, bent and twisted, it becomes 

 compactly folded within the narrow confines of the shell. 



In the common fowl the heart begins to beat at the begin- 

 ning of the second day, and earlier in most wild birds. A pro- 

 tective water-jacket is early 

 formed around it, and a tem- 

 porary "lung" is developed. 

 The latter is in the form of a 

 thin -walled sac, which closely 

 underlies the shell. Mean- 

 while an air-chamber has 

 slowly made its appearance 

 at the larger end of the egg, 

 between the hard shell and 

 underlying membrane, in 

 consequence of evaporation 

 of the white ; into this the bill 

 of the little bird is unerringly 

 thrust, and with its stout 

 spur or "egg-tooth" is soon 

 pressing against the shell. 

 Invigorated by the air which is now drawn into the true lungs, 

 the chick presses hard against the walls of its prison, and its 

 first muffled bleats are faintly heard. The egg is starred, 

 pipped, and finally cracked open, usually into two unequal 

 halves, by a fissure extending around the larger end. Wet and 

 trembling, the little chick slowly crawls forth into the world 

 of light and action, and its new life begins. 



The whole process of development in the egg is wonderfully 

 rapid, lasting from two weeks or less in the common land birds 

 to seven weeks in the Ostrich. 



The cast-off shells are carefully removed by many birds and 



Fig. 104. Egg of the Great Herring Gull 

 with part of the shell chipped away to show 

 the chick inside. The bill is thrust into the 

 air-chamber at the larger end, and bears a 

 large spur or "egg-drill." The head is twisted 

 about so as to lie under the right wing, and 

 the remains of the yolk-sac can be seen be- 

 neath the folded webbed feet. 



