VI II 



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, 



Fig. 66. — Egg, after Fester and Balfour, a, air-chamber between two layers of 

 shell-membrane ; CH, chalaza; ; E, embryo ; is, internal shell membrane ; WY, white 

 yolk ; yy, 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 



