276 THE STRUCTURE AND LIFE OF BIRDS chap. 



within the thin membrane that envelopes the yolk is 

 sometimes at least next to the breast. Break a hole 

 about half an inch square in the side of a hen's egg, 

 and on the yolk will be seen a round, whitish spot. 

 If now the hole be walled up with sticking plaster so 

 that the white will not run out, and, the egg having 

 been turned over, another similar hole be made on the 

 other side, the whitish spot will again be seen. The 

 yolk has rolled over, as it always does when the egg 

 is turned. The white spot is the place where the 

 embryo lies, and it is always at the top, close to the 

 body of the bird. If the eggs under a sitting hen 

 are marked and examined the day after, the main 

 object with which she moves them will soon become 

 apparent. It will be found that she has shifted the 

 outside ones to the middle, and the middle ones to 

 the outside, so that all may have a turn at the warmest 

 place and consequently hatch about the same time. 

 For several days I marked the eggs in two nests, in 

 which they lay in a circle with three in the middle. 

 As a rule only two of these three (for no system 

 works perfectly) had been replaced by others from the 

 outer ring. 



Now for the cause of the yolk's rolling over when 

 the egg is turned. When you break a raw egg, you 

 can hardly help seeing two twisted cords, of an 

 opaque white, which at their ends spread out and lose 

 themselves in the albumen. It has been supposed, 

 and in some recent books on natural history it is 

 still maintained, that these cords are the machinery 

 by which the revolution is brought about. But since 

 they only float in the albumen, it is difficult to see 



