Incubation 205 



From this time until the middle of the nineteenth 

 century, comparatively little advancement was made 

 in the construction of incubators. Since 1870 many 

 improvements have been made and a large number 

 of incubators have been placed on the market. 



In the manufacture of the modern incubator, 

 the idea has been largely to imitate nature. As 

 the hen applies heat above the egg, so nearly all 

 incubators apply heat to the egg-chamber from 

 above. In natural incubation the hen provides 

 a nest or shallow receptacle for the eggs, usually 

 on the ground. After laying a number of eggs, 

 the hen becomes "broody" and commences to sit, 

 or incubate. At this time the underpart of the 

 hen's body is more abundantly supplied with blood 

 than during the laying or molting periods. In 

 other words, what is known as the organ of incu- 

 bation, or plexus, most fully develops at this time. 

 This consists of a network of arterial blood-vessels, 

 near the surface of the body, which bring greater 

 heat to the eggs than would otherwise be supplied. 

 In order to perfect systems of artificial incubation, 

 many persons have made a close study of the natural 

 method and have been led to adopt some devices 

 or practices which were accidental or necessary 

 to the hen, but not essential to perfect incubation. 

 The fact that hens sometimes leave the nest in the 

 early morning to search for food, when the grass is 

 still wet with dew, and upon returning moisten 



