REQUISITES OF A GOOD INCUBATOR. 275 



that the fluids in the embryonic structures are ample to keep the mem- 

 branes moist up to the time of exclusion, and the rate of evaporation 

 is not the same under any two degrees of temperature." 



THE MOTHER HEN THE PATTERN. 



What are the natural processes ? The hen's nest is 

 concave to keep the eggs close together, and shallow 

 enough to prevent them from lying two deep, thus 

 bringing the upper part of each egg containing the germ 

 in close contact with her body or the feathers next her 

 skin. Other feathers, especially those of her wings, are 

 distended so as to form a wall, enclosing the egg on all 

 sides and retaining the heat, the construction of the 

 feathers being such that while all strong currents of air 

 are prevented, yet the slightest movement of the hen 

 causes the elastic down to operate like fans and drive 

 out air from the nest, to be replaced by fresh air from 

 outside. Indeed, there is a slight, exceedingly gentle 

 circulation of air going on, strained through the laby- 

 rinth of the overlapping feathers, even when the 

 hen is asleep. Also, through the natural law of diffu- 

 sion, the poisonous gas thrown off from the embryo 

 through the porous egg shell is forced out of the nest 

 through the feathers independent of any circulation of 

 air. It does not stay to become accumulated under the 

 hen to the injury of the incipient chick ; for the law 

 above hinted at compels it to diffuse itself in all direc- 

 tions, and it will overcome gravity and rise, though 

 heavier than the air with which it mingles, and will force 

 itself through feathers as it cannot do through the 

 walls of an incubator. 



As the eggs at the middle of the nest become very 

 warm to the touch of the hen she pushes them away by 

 hooking her beak and the upper portion of her neck 

 over those at the outside and pulling them along to take 

 the place of the former. The operators in the Egyptian 



