240 POULTRY CULTURE 



an egg is fertile varies with the vitality of the germ, the color and 

 texture of the shell of the egg, and the intensity of the light 

 before which it is observed. A thin-shelled white egg in a strong 

 light may show fertility inside of twenty-four hours. A dark-shelled 

 egg, weak in fertility, tested in a poor light, may appear doubtful 

 after a week of incubation. Ordinarily tests made at the fifth to the 

 seventh day give an experienced operator reliable indications of 

 the fertility and vitality of eggs that have been incubated under 

 proper conditions. Though not invariable, it is the general rule 

 that the fertility of eggs from a mating is quite constant through 

 a season ; so that when the degree of fertility of eggs from a 

 pen, flock, or stock is once found, it is likely to be maintained for 

 some time. 



As a rule, fertility and vitality reach their highest point of com- 

 bination at the natural hatching season. Fertility is lowest and 

 vitality highest in advance of this season, and fertility highest 

 and vitality lowest after it; but numerous special cases furnish 

 exceptions to these general conditions. Fertile, hatchable eggs are 

 the prime factor in incubation, and a knowledge of the hatching 

 properties of the eggs used is absolutely necessary for intelligent 

 judgment of other factors when hatches are unsatisfactory. Self- 

 evident as this seems when stated, a great deal of work in incuba- 

 tion is done without this basic knowledge, the operator working quite 

 in the dark. Detailed instructions as to determination of fertility 

 are given in the paragraphs relating to the operation of incubators. 



Heat the energetic factor in incubation. Given a hatchable 

 egg, the continuous application of a proper degree of heat for 

 a definite period of time, varying in different kinds of birds, will 

 produce an embryonic bird which, when it has attained the fullest 

 possible development within the shell, will break the shell and 

 emerge from it. In nature the heat for incubation is usually applied 

 by the bird which laid the egg, relieved at intervals, perhaps, by 

 its mate. In artificial incubation, oil, coal, gas, and electricity are 

 used. The source of heat, however, is immaterial. All that is neces- 

 sary is that the proper degree of heat be continuously maintained 

 (not absolutely, but approximately) for the required time, under such 

 circumstances that atmospheric conditions affecting the develop- 

 ment of the embryo within the egg will not be markedly unfavorable. 



