104 A COURSE ON ZOOLOGY. 



furnishes the rudimentary substance for the mineral mat- 

 ter of the bones. The air which must be supplied for 

 the respiration of the embryo passes through the pores 

 of the shell, and there is at the large end of the egg a 

 sort of reservoir, the air-chamber, contained between the 

 two layers of the chorion. Air is indispensable for the 

 development of the germ. When eggs are to be pre- 

 served for food they are dipped in a solution of lime or 

 some other substance that will close up the pores of the 

 shell. 



The constituent parts of the egg are not all formed in 

 the same organs. The yolk or vitellus is organized in 

 the ovaries, which form a bunch suspended from the 

 posterior walls of the abdomen of the bird. When de- 

 tached from the ovary, the yolk descends towards the 

 cloaca through a canal called the oviduct; here it be- 

 comes covered with the albumen, and as the yolk has in 

 its descent a motion of rotation on its axis, there results 

 a torsion that forms the albuminous ligament. In the 

 lower parts of the oviduct are secreted the calcareous 

 matter of the shell and the coloring substances that in 

 certain species tint the surface. The egg is then passed 

 into the cloaca and is expelled. 



In order that the egg may be hatched it must for 

 a certain length of time be maintained at a temperature 

 of about 107, and for this reason the mother sits on it. 



In the domestic state the cock does not help the hen 

 either in the cares of incubation or in the rearing of the 

 young family, but he intrepidly defends his hens and 

 chicks from all attacks and dangers. 



