Formation of the Egg. 
37 
eggs aie of different sizes, according to the different degree of development, and during 
the peiiod of laying are constantly coming to maturity in due succession, so as to keep up the 
supply of eggs with which we are so familiar. It is worthy of remark that from the germ of 
every egg a narrow passage or canal runs to a small chamber in the centre of the yolk, which 
can be readily seen if the yolk of a hard-boiled egg be cut across. 
As the yolk becomes fully matured, the enclosing membrane or ovisac becomes thinner 
and thinner, especially round its greatest diameter or equator, which then exhibits a pale zone 
or belt called the stigma. Finally fecundation takes place, the sac ruptures at the stigma, 
and the liberated yolk and germ, surrounded by a very thin and delicate membrane, is received 
by the funnel-shaped opening of the oviduct or egg-passage, whose 
office it is to convey it to the outer world, and on its way to clothe 
it with the other structures needful for its development and preser- 
vation. This organ, with its various convolutions a little modified 
for convenience of representation, is shown at Fig. 17, and in an 
ordinary hen is nearly two feet in length. It will easily be seen 
that not unfrequently two yolks may become detached and enter 
the oviduct at nearly the same time ; in which case they are very 
likely to be enveloped in the same white and shell, causing the 
“ double egg ” so well known to every poultry-keeper. 
Thus received into the oviduct, the yolk becomes enveloped 
hi a glairy fluid called the white, or by chemists albumen. This 
is secreted by the mucous membrane of the oviduct, and added 
layer by layer as the egg passes on. These different layers can 
be easily seen, and even peeled off in succession when an egg is 
hard-boiled. The uses of the white or albumen are manifold. It 
is eminently nutritious, forming, indeed, the chief nourishment of the 
chick during its growth in the shell ; as it becomes absorbed by 
the little animal, and forming as it does by far the greater part 
of the egg when laid, it gives the fast-growing little body the needed I7 
increase of room ; it is a very bad conductor of heat, and hence 
guards the hatching egg against the fatal chills which would otherwise occur when the hen left 
the nest ; and finally, it preserves the still more delicate yolk and vital germ from concussion 
or other violent injury. The manner in which the last purpose is effected is very beautiful. 
Besides the ordinary white, two longitudinal cords or strings of much denser and even slightly 
fibrous albumen are formed, which are easily distinguished if an egg be broken into a basin. 
These cords are termed the chalazce, and are attached in a spiral form to the under side of 
the yolk, to which they therefore serve as ballast or weights, and always keep the germ uppermost, 
where it can best receive the heat from the sitting hen. 
At a still further point of the oviduct the egg becomes invested with the skin or parchment- 
like covering which is found inside the shell, and is called by physiologists the membraua 
putaminis. In reality this skin consists of two layers, which can easily be separated ; and in fact 
at the laree end of the egg they do separate entirely, forming what is commonly called the 
air-bubble, or by anatomists the vesicula asris. How formed is still a mystery ; but it is an 
ascertained fact that the air in this bubble or chamber contains a far larger portion of oxygen 
than the atmosphere. At first the chamber does not exceed a quarter of an inch in diameter, 
but as the egg gets stale it becomes larger and larger, so that even in eggs stored it fills 
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