492 A STUDY OF FARM ANIMALS 



reproduction, although for food it has equal value with the 

 fertile one. The fertility of the egg can not be determined 

 except by incubation. After the egg has been under the 

 hen for five to seven days, ordinarily one may easily tell 

 whether it is fertile or infertile. If infertile, it will appear 

 clear and show none of the changes subsequently described. 

 The testing or candling of eggs is a simple process of 

 looking through the egg with the aid of special light. One 





Figure 235. A home made egg candler. Reproduced from Farmers' Bulletin 

 No. 1040, United States Department of Agriculture. 



may take a piece of common cardboard, one side of which is 

 black, in which is cut an oval hole not quite as large as an 

 egg. If the cardboard is held before a lighted lamp in a 

 dark room, blackened side towards one, and an egg is held 

 in the hole, the one that contains a chick will appear dark 

 and opaque except at the larger end, while a sterile egg will 

 be clear and show light. In the trade, where all eggs are 

 examined before a light, this process is known as candling. 

 Black lamp chimneys with holes in them are made for use 



