116 TNTRODUCTION TO ZOOLOGY. 



covering, and the largest flying bird is much smaller 

 than the largest land or water animal. 



The eggs of birds are remarkable for their very 

 large size in comparison with those of other animals. 

 The greater part of the egg consists of food for the 

 embryonic bird. The egg begins to divide at the nu- 

 cleus, as all other eggs do, but the planes of seg- 

 mentation do not extend entirely through the body of 

 the egg, as they do in the eggs of the frog and of some 

 other animals. The segmentation is confined to a 

 small portion of the egg which is called the germinal 

 disc, and the body of the bird is foi-med from the 

 cells thus produced. The remainder of the egg is 

 absorbed as food, or nourishment, by the embryonic 

 bird. 



This kind of segmentation is called partial, or 

 meroblastic segmentation. 



The usual method of incubation is for the bird to 

 sit upon the egg. The side of the egg upon which the 

 germinal disc, or white spot, lies is apparently of less 

 specific gravity than the opposite side. This is shown 

 by the fact that in about nine out of every ten eggs 

 that are broken, the germinal disc appears upon the 

 upper side, and it takes careful and persistent effort 

 to turn the yolk over so that the germinal disc will 

 not sliow. The germinal disc is upon the upper side 

 of the egg. This position brings it nearest the body 

 of the sitting bird, which is the source of warmth, and 

 this is the most favorable condition for the growth 

 of the embryo. 



An air space is usually found in an egg. The air 

 space arises in the following way : When the egg is 

 laid, it is of the same temperature as the body of the 

 bird. It very soon cools, and its contents are thereby 

 contracted. Tlie shell, however, being rigid, cannot 

 contract, so air is drawn in through the pores of the 

 shell. The air space is found in the large end, because 



