322 FARM ANIMALS 



In some comparative incubation tests made in 

 studying this point it has been found that fully 

 three times as many infertile eggs were laid by hens 

 closely confined as by those which had an unre- 

 stricted range. Another serious cause of infertility 

 in eggs is overfeeding. Hens which are excess- 

 ively fat are likely to lay a high percentage of eggs 

 with weak germs or absolutely infertile. 



A distinction is usually made between fertility, 

 strength and vitality of eggs. The egg which 

 begins development after incubation is said to be 

 fertile although it may not result in a completely 

 formed chicken. For practical purposes, however, 

 there is no difference in the results whether the 

 egg is absolutely infertile or whether the chicken 

 dies before the end of the twenty-one days. In 

 practice it has been found that chickens in eggs 

 with weak germs die between the tenth and twenty- 

 first days of incubation. Another cause of infer- 

 tility is found in allowing hens to lay for too long a 

 period before taking eggs for hatching. In hens 

 which have laid industriously through the whole 

 winter season the spring eggs show a low percentage 

 of fertility until the hens have been allowed the 

 freedom of an outside run for a week or so. 



The fertility of eggs varies from zero to one 

 hundred per cent. In a long series of tests in Rhode 

 Island eggs obtained from different sources and 

 under different conditions proved to be fertile to 

 the extent of thirty-five per cent., forty-one per 

 cent., forty-four per cent., and eighty-three per 

 cent, under different conditions. A fertility of 

 eighty per cent, is considered quite satisfactory. 

 Eggs which will hatch under a hen will also hatch 

 in the incubator. This point has been determined 

 by numerous comparative tests. The incubator, 



