. The structure and life of birds . rfectly) had been replaced by others from theouter ring. Now for the cause of the yolks rolling over whenthe egg is turned. When you break a raw egg, youcan hardly help seeing two twisted cords, of anopaque white, which at their ends spread out and losethemselves in the albumen. It has been supposed,and in some recent books on natural history it isstill maintained, that these cords are the machineryby which the revolution is brought about. But sincethey only float in the albumen, it is difficult to see VIII THE BIRD WITHIN THE EGG 277 how they can help towar
. The structure and life of birds . rfectly) had been replaced by others from theouter ring. Now for the cause of the yolks rolling over whenthe egg is turned. When you break a raw egg, youcan hardly help seeing two twisted cords, of anopaque white, which at their ends spread out and losethemselves in the albumen. It has been supposed,and in some recent books on natural history it isstill maintained, that these cords are the machineryby which the revolution is brought about. But sincethey only float in the albumen, it is difficult to see VIII THE BIRD WITHIN THE EGG 277 how they can help towards it. They have a use, forthey act as buffers and save the yolk from violentconcussion when the egg is shaken. The turning isdue simply to the fact that the yolk is lighter on thatside on which the embryo lies. If you examine ahard-boiled egg, you will see that the yolk is notuniform in colour ; most of it is yellow, the rest iswhitish yellow, the two being different not only incolour, but in microscopic structure. The white yolk,. Fig. 66.—Egg, after Fester and Balfour, a, air-chamber between two layers ofshell-membrane ; CH, chalaza; ; E, embryo ; is, internal shell membrane ; WY, whiteyolk ; yy, yellow yolk. as it is called, lies under the round white spot, and,swelling out like a flask, descends to the centre of thesphere. When the egg is turned, the yolk becomes top-heavy, and therefore rolls over. As yet this is onlya rough description. Besides the flask-shaped massof white yolk, there are four very thin layers dividingthe yellow into five. There is another feature that belongs only to anegg that is not perfectly fresh—the chamber at thelarge end. This appears after a time, even if the egg 278 THE STRUCTURE AND LIFE OF BIRDS chap. is put under a hen, and gradually increases in size asthe white shrinks by evaporation. To follow in detail the kaleidoscopic developmentof the embryo from day to day, till at last by thehelp of the egg-tooth at the end of his beak hepecks
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