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uppermost, so as to receive the heat from the sitting hen. The mechanism 

 by which this is managed is very simple : The lower side of the yolk is 

 weighed or ballasted by two heavy twisted masses of very firm albumen, 

 which keep the germ constantly uppermost. Contrary to general belief, 

 these ballasting weights are found in all eggs, whether laid by pullets or old 

 hens. If an egg has been set upon for even a few hours, the size of the 

 germ is increased, 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. 

 The yolk, like the white, is composed of concentric layers, which may be 

 seen when it is boiled hard, and from the germ a tube runs to a central 

 hollow or cavity, often noticeable 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 one hundred degrees, the germ undergoes a remarkable series of 

 alterations, being gradually developed into the perfect chick. During the 

 period of incubation, various changes occur. 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 its development, the 

 chicken derives its nourishment 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 

 peries of vessels in a temporary respiratory membrane which lines the porous 

 shell ; this makes its appearance on the third day, and gives rise to that 

 opacity of the fertile egg which may always be observed. It is not until the 

 nineteenth day of incubation that the beak of the chick ruptures the enlarged 

 air vesicle, and it then only commences to breathe by means of its lungs. 

 This is accompanied by a peculiar sound known as " tapping," which is 

 merely respiratory, and is not caused by contact of any kind between the 

 beak of the chick and the interior of the shell. 



