THE POULTRY BOOK. 
27 
white. In breaking a number of eggs into a basin there may be observed a small 
circular speck on each yolk. This speck is the rudiment of the young chick, and 
the construction of the egg is such that on whichever side it is turned the rudi- 
mentary germ is always uppermost, so as to receive the heat from the breast of 
the sitting hen. The mechanism by which this is managed is very simple : the 
lower side of the yolk is weighted or ballasted by two twisted heavy masses of very 
firm albumen, termed the chalazae, which, hanging down, keep the germ constantly 
uppermost, on whichever side the egg may be laid. Contrary to general belief, 
these ballasting weights are found in all eggs, whether laid by pullets or old 
hens. 
If an egg has been sat on, even for a few hours, the size of the germ is in- 
creased, and if left in the nest of a sitting hen for twenty-four hours, small blood- 
vessels may be seen, forming a beautiful zone around it. It should not be 
imagined that the yolk is a mere unorganized liquid ; it is composed, like the 
white, of concentric layers, which may be seen when it is boiled hard ; and from 
the germ a tube runs to a central hollow cavity — that is also often to be noticed 
when an egg boiled hard for salad is cut across. 
When a fecundated egg is placed under a hen, or deposited in an incubator and 
subjected to a temperature somewhat above 100° Fahrenheit, the germ undergoes 
a remarkable series of alterations, being gradually developed into the perfect chick. 
During the period required for the complete development, various chemical and 
physical changes occur, the most important of which may be briefly alluded to. 
The air-vesicle at the end gradually becomes larger in proportion as the water of 
the albumen evaporates through the pores of the shell. During the development 
of the chicken, its nourishment is derived chiefly from the yolk ; and shortly 
before birth the remainder of the yolk is drawn into the abdomen, and passing 
into the digestive canal, constitutes the first food of the newly-hatched animal. 
During incubation, the blood of the chick is aerated by passing through a series 
of vessels in a temporary respiratory membrane which lines the porous shell ; this 
makes its appearance on the third day, and gradually extending, gives rise to that 
opacity of the fertile egg which has been already alluded to. It is not until the 
nineteenth day of incubation that the beak of the chick ruptures the enlarged air- 
vesicle, and it then first commences to breathe by means of its lungs. This is 
accompanied by a peculiar sound ; and we feel much indebted to the late Dr. F. K. 
Horner, of Hull, for the following abstract of a paper on this subject, which was 
read by him before the British Association for the Advancement of Science : — 
“I think the Poultry Book- the fittest medium for the promulgation of a fact, 
or rather discovery, on a subject concerning which views altogether erroneous 
prevail ; I mean the manner in which the chick first breaks, and liberates itself 
from, the shell. 
‘‘It is universally believed that the continued ‘ tapping ’-like sound, so percep- 
tible within the egg, is produced by the bill of the bird constantly striking or 
coming in contact with the shell in its efforts to break it. Indeed, philosophers 
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