THE INCUBATION OF THE EGG 163 



per cent, of the total weight of the egg are (1) the yolk, 32.75 

 per cent.; (2) the albumen, 57.01 per cent.; (3) the shell 

 membranes, 0.25 per cent., and (4) the shell, 9.99 per cent. 



The yolk and its " germ spot," known as the blastoderm, 

 are inclosed in a delicate transparent membrane called the 

 vitelline membrane. This membrane is responsible for the 

 yolk maintaining a spherical shape. The material making 

 up the body of the yolk is a highly nutritious food which is 

 gradually digested and absorbed by the growing embryo. 

 It has a rather definite structure, being divided into yellow 

 and white yolk. As shown in Fig. 76, the yellow yolk makes 

 up the bulk of yolk material. The most of the white yolk is 

 found in the flask-shaped mass, situated at the centre of the 

 yolk sphere, with the neck extending toward and flaring out 

 just beneath the blastoderm and entirely surrounding the 

 yolk just inside the vitelline membrane. There are said to be 

 several thin concentric layers of white yolk throughout the 

 yolk mass (Fig. 76), but these are difficult of demonstra- 

 tion. 



In an egg that has not been moved for some, time the yolk 

 will be found to be floating on the albumen near that portion 

 of the shell which is then uppermost and with the blastoderm 

 uppermost. If the egg is turned the yolk will gradually 

 return to the same relative position. This is caused by 

 the lower specific gravity of the yolk as compared with the 

 albumen, and of the hemisphere of the yolk supporting the 

 blastoderm as compared with the opposite one. 



It has been suggested that nature's purpose in this arrange- 

 ment is to bring the blastoderm as close to the source of heat 

 during natural incubation as possible. 



According to Lillie,' " Both kinds of yolk are made up of 

 innumerable spheres, which are, however, quite different in 

 each.. Those of the yellow yolk are on the whole larger 

 than those of the white yolk, with extremely fine granular 

 contents. 



Immediately surrounding the yolk and adhering closely 

 to the vitelline membrane is a layer of very dense albumen 



' Developmept of the Chick- 



