Vil THE BIRD WITHIN THE EGG 277 
how they can help towards it. They have a use, for 
‘they act as buffers and save the yolk from violent 
concussion when the egg is shaken. The turning is 
due simply to the fact that the yolk is lighter on that 
side on which the embryo lies. If you examine a 
hard-boiled egg, you will see that the yolk is not 
uniform in colour; most of it is yellow, the rest is 
whitish yellow, the two being different not only in 
colour, but in microscopic structure. The white yolk, 
Fic. 66.—Egg, after Foster and Balfour. a, air-chamber between two layers of 
shell-membrane ; cH, chalazz ; £, embryo; 1s, internal shell membrane ; wy, white 
yolk ; vy, yellow yolk. 
as it is called, lies under the round white spot, and, 
swelling out like a flask, descends to the centre of the 
sphere. When the egg is turned, the yolk becomes top- 
heavy, and therefore rolls over. As yet this is only 
a rough description. Besides the flask-shaped mass 
of white yolk, there are four very thin layers dividing 
the yellow into five. 
There is another feature that belongs only to an 
egg that is not perfectly fresh—the chamber at the 
large end. This appears after a time, even if the egg 
