132 The Hen at Work 



cover the membrane and form the shell. This 

 fluid is white, and is chiefly carbonate of lime. 



In the natural state a fowl will pick up enough 

 material to form this fluid for a few eggs in the 

 spring, but when eggs are expected at all seasons, 

 special supplies must be provided — oyster or clam 

 shells, which were themselves formed of just such 

 a fluid. In the digestive tracts of the hen the 

 shell returns to a liquid state, and hardens again 

 after it has formed around the egg. 



The egg remains in the lower part of the oviduct 

 about twelve hours, while the vshell is becoming 

 solid. It is then expelled by muscular contractions, 

 narrow end first, and the process begins again at 

 the upper end of the oviduct, if the hen is in active 

 laying condition. 



The normal time consumed in finishing the egg 

 is about twenty-four hours. This is frequently 

 extended to a longer period. On the other hand 

 a hen may lay one egg early in the morning and 

 another before going to roost, if she is prolific, 

 and the eggs happen to come along at just the 

 right time. 



General Principles of Incubation. — ^The prob- 

 lem of incubation is to place the egg under such 



