THE VEETEBEATES 



447 



velopment may start again if the egg be warmed to the proper 

 temperature. In natural conditions this is provided for by 

 the instinct which causes female birds (sometimes the males) 

 to sit on or brood the eggs. The feathers prevent rapid loss 

 of the heat afforded by the warm ventral surface of the body 

 of a brooding bird. This brooding instinct usually appears 

 soon after a female bird has laid the eggs which in a given 

 season have developed in the ovary. In many wild birds 

 such a season of egg-laying comes only once a year ; in some 

 species two or three broods of eggs may be laid in a summer ; 

 and the well-fed domesticated hen may lay from 100 to more 

 than 200 eggs per year, if not allowed to waste time and 

 energy by brooding after each set of 10 to 20 eggs, as they 

 instinctively do. The eggs of birds are large because they 

 have a great store of food (yellow " yolk," and the " white " 

 or albumen) for nourishment of the embryo during the de- 

 velopment. The eggs in the ovaries of young birds are small 

 spherical cells, but as they mature the storage of food causes 

 enlargement. For exam- 

 ple, in an ordinary hen's 

 egg the " yolk " with its 

 inclosing yolk-membrane 

 is about one inch in diam- 

 eter, but in a young ovary 

 it is a microscopic cell. 

 The " white " or albumen 

 which surrounds the yolk 

 and also the shell are se- 

 creted around the egg as 

 it passes through the ovi- 

 duct on the way to the 

 exterior. Obviously, the 

 " yolk " is the real egg, corresponding to a frog's egg, and the 

 " white" and shell are later additions formed like the jelly 

 around frog's eggs. 



FIG. 151. Diagram of bird's egg. yk, 

 yolk ; alb, white or albumen ; bl, germ- 

 disc ; ch, thickened albumen which 

 holds yolk in position ; sh.m, two 

 shell membranes. (From Parker and 

 Haswell) 



