BIRD HABITS 299 



yolk and white both furnish food for the developing embryo, 

 somewhat as does the endosperm of a seed, while the membranes 

 and shell are protective coverings, porous enough to admit air to 

 the chick, and to allow the discharge of carbon dioxide. Fertiliza- 

 tion takes place in the ducts leading from the ovaries. Cell division 

 goes on for about twenty-four hours and then ceases, only to 

 recommence in case the egg is warmed and kept at proper 

 temperature. 



As the tiny egg germ passes along the oviduct, the yolk and white 

 are added, layer by layer; these layers sometimes separate in a 

 hard boiled egg. The yolk is the real egg, corresponding to that 

 of fish or frog, while the white and shell are added nourishment 

 and protection somewhat like the jelly that coats the frog and toad 

 eggs. 



Decay of stored eggs is caused by bacteria that pass through the 

 pores of the shell. If eggs that have no bacteria in them (i.e. 

 " fresh ") are sealed air-tight by a solution of water glass, they 

 do not decay as no bacteria can get in. If eggs are kept in cold 

 storage, the bacteria, even though present, do not develop and 

 the egg " keeps " for months with but little change. 



The shape of most eggs is oval, for two reasons: they pack 

 better together in the nest, and cannot easily be rolled out. Try 

 to roll a bird's egg and it will follow a circle and come back to 

 about where it started. Eggs of birds making deep, safe nests 

 are not so oval, partly because they are safe without this adaptation. 



The number of eggs varies with the amount of care that the 

 parent birds can give the young. It is greatest in those kinds, 

 whose young receive the least attention and which try to shift for 

 themselves early in life. This increases their chances of destruc- 

 tion and makes necessary more eggs if any are to survive. In case 

 of birds that are helpless when hatched and are fed and protected 

 by parents, the number is lower. Common wood and field birds 

 average about five, while game and river birds have twelve or 

 more; on the other hand birds of prey produce but one or two. 



The size of the egg is greater in those species which hatch well 

 developed, since more stored food is required to carry on the longer 



