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works perfectly) had been replaced by others from the 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 
