992 Marvels of the Universe 
This is lined with a delicate, very tough, double-layered skin at the broad end of the egg: as 
incubation proceeds, the inner layer comes away from the outer, and the space formed becomes 
filled with air to enable the young chick to breathe. The air finds its way through the shell, which 
is porous, and composed of two layers, a very thin outer layer covering a thicker inner layer, 
which is pierced by numerous vertical canals. This shell is formed by the walls of the 
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oviduct as the mass of yolk and its enveloping white’ passes down, and, in some birds, 
as in hawks and guillemots, for instance, rich pigments and curious patterns are added to its 
outer surface. 
Now as to the coming of the chick. After about sixteen hours’ incubation the circular disc 
or “ germ” which we have already seen, becomes pear-shaped, and grooved down its middle ; and 
the foundations of its backbone and nervous system are laid. By the first half of the second day 
the heart and principal blood-vessels have put in an appearance, though as yet, of course, they are 
but imperfectly developed, and towards the end of the day two structures peculiar to embryonic life 
give their first signs of growth. These are the amnion and the allantois. The first-named eventually 
grows into a delicate transparent bag filled with a clear 
fluid, and surrounding and enclosing the embryo, pro- 
tecting it from injury. The allantois is a bag which 
grows out from the hinder end of the gut and receives 
the waste given off by the developing kidneys. But 
more than this, its walls growing upwards, spread 
under the shell, and become richly supplied with blood- 
vessels. These, absorbing the air which passes through 
the shell, serve to oxygenate the blood, so that the 
allantois really does the work of the lungs which have 
not yet come into being. 
During the third day the brain, the eyes, the nerves, 
the lungs, liver, and other glands begin to appear. 
And by now, be it noted, the “ white” of the egg has 
largely disappeared, having been absorbed by the 
‘ 
Photo by] } LW. B. Randles. blood-vessels. 
HEN’S EGG. During the fourth day the nose, ears and jaws 
Aftereightcen} hours) incubation?) Tihejzcrm begin to take shape; the wings and legs appear in 
is seen to be changing to a pear shape. 
the form of tiny buds, like large pins’ heads. Twenty- 
four hours later these limbs have lengthened, elbow and knee can now be recognized, and faint 
traces of digits are visible enclosed between a web. 
The sixth day marks a new epoch. Till now the growing body had not developed sufficiently 
far to enable one to trace any likeness to any particular animal, much less to a bird, but now the 
bird-like features are unmistakable. Yet the head is still huge in proportion to the rest of the body 
and there is no beak, which does not take shape till the twelfth day. The day following nails appear 
on the toes and scales on the legs. 
As early as the ninth day feathers begin to make their appearance, in the form of tiny button- 
like, or stud-like projections over the body, and by the thirteenth day these have assumed the 
form of long rods, which soon will burst, and release the nestling down. 
By the nineteenth day our developing chick has become a “ ripe embryo.” The white of the 
egg has entirely disappeared, and the remains of the yolk are very fluid and much diminished. Just 
before hatching, which is now imminent, the rest is drawn up into the body, and stowed away under 
the intestines. The long stalk, or neck of the yolk bag, is in open communication with the gut, and 
through this passage the last of the yolk is taken up. Hence it is that for some twenty-four hours 
after hatching the young chick requires no food. He still has the remains of “ his bottle ” to finish! 
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