212 A HISTORY OF BIRDS 



white and the shell are accessory structures, though it is to be 

 noted the former is finally absorbed by the developing chick. 



These accessoiy portions — ^the albumen, shell membrane 

 and shell — are added in the lower part of the oviduct, the mem- 

 brane lining the shell being formed by the transformation of 

 the outermost layer of albumen, while the shell itself is formed 

 by the secretion of a thick white fluid, in which, finally, the pig- 

 ment, when this is present, is mingled. The albumen and the 

 shell membrane are formed in about three hours, while the 

 completion of the shell takes about eighteen hours. 



With regard to the membrane adherent to the shell we 

 may remark that this is double, and that at the larger end of the 

 egg the two layers part company, the space between being filled 

 with air drawn in through the porous shell. As incubation pro- 

 ceeds this space grows larger, and thus the necessary supply of 

 oxygen for the developing chick is obtained. 



Such then is the answer to the questions, what is an egg ? 

 and what is this " something more " ? 



The fertilisation of the egg takes place immediately after its 

 escape from the capsule in which it was developed, into the 

 oviduct ; so soon as this is eff"ected the early stages of develop- 

 ment begin, but these cease so soon as the egg is laid and be- 

 comes cold, to be resumed again by the brooding of the parent. 



