176 POULTRY PRODUCTION 



The principal divisions of tlie egg, and their weights in 

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

 ])er cent; (2) the white, 57.01 per cent; (3) the shell iiiem- 

 l)ra,iies, 0.25 per cent, and (4) the shell, 0.99 ])er cent. 



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

 are inclosed in a delicate transparent menihrane called the 

 vitelline membrane. Tliis membrane is responsible for the 

 yolk maintaining a spherical sliape. Tlie 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 

 anfl white yolk. As shown in Figure 82, tlie 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 center 

 of the yolk sphere, with the neck extending toward and flaring 

 out just beneath the blastoderm and entirely surrounding the 

 yolk just inside the \'itelline membrane. There are said to be 

 se\cral thin concentric layers of white yolk throughout the 

 yolk mass (Figure 82), l>ut these are sometimes difficult of 

 demonstration. 



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

 will be found to be floating on the white 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 hemis])here 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 

 innumeral)le spheres, which are, however, quite difi'erent in 

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

 than those of the white yolk, with extremely fine graiuilar 

 contents." 



Immediately surrounding tlie yolk and adhering closely 



' Development of the Chick. 



