494 A STUDY OF FARM ANIMALS 



"When ready to come out, the chick raises its head and pierces the 

 inner shell membrane, and immediately starts breathing the air in the 

 chamber, which causes the pulmonary circulation to become active and 

 the embryonic circulation to cease. The head is next raised into the air 

 chamber, and the chick deals blows upon the shell, which, when often 

 repeated in the same place, result in fracturing it. This process is re- 

 peated until the shell is broken around about one third of the way from 

 the large end. The chick then presses its head against the large end 

 and its feet against the small end, and then by pushing is able to throw 

 off the shell lid and make its exit." 



The incubator is a box-like device containing a space in 

 which eggs may be incubated by means of artificial heat. 

 The hatching of eggs by artificial incubation has been in 

 operation for thousands of years, especially in Egypt and 



Figure 237. A pair of vigorous day-old chicks. Photograph from Prof. 

 F. S. Jacoby. 



China. There are various designs of incubators made, rang- 

 ing in size from those which contain but a few eggs up to 

 those with a capacity for thousands. Incubators in use at 

 the present time are heated by hot air from a kerosene 

 lamp or by a hot-water system. The hot-air type is the one 

 in more common use. The eggs, one layer deep, are placed 

 in movable, wire-bottomed trays. The temperature of the 

 incubator is regulated by the automatic action of an instru- 

 ment called a thermostat, which is sensitive to heat changes. 

 This instrument is set so as to reduce or increase automatic- 



