Incubation. — Part II 1391 



incubator should be run with only slight variations. Although it is best 

 to maintain an even temperature, it is not always possible to do so, and 

 an occasional variation of one half degree, or slightly more, will not result 

 seriously if the average temperature is correct. A high temperature 

 should be avoided, especially at the beginning of incubation. The tem- 

 perature should be read through the glass door, which should be opened as 

 little as possible. 



Temperature, first week. — The position of the thermometer should 

 always be considered in determining the proper temperature to maintain. 

 If the thermometer hangs above the trays, as it does in some incubators, 

 thereby registering the temperature of the air and not that of the eggs, 

 the actual temperature of the eggs is one to one and one-half degrees lower 

 the first week than the registered temperature. Therefore, in order to 

 give the eggs the proper amount of heat the first week, when hanging 

 thermometers are used, it is necessary to keep the temperature at 102.5'* 

 or 103° F. ; with contact thermometers, the temperature should be 102°. 

 Contact thermometers should always be placed between two fertile eggs. 



Temperature, second week. — The temperature within the incubator is 

 less influenced by that outside after the first week, owing to the increasing 

 amount of animal heat given off by the growing embryos. With hanging 

 thermometers the mercury should be held at 103° F., while with contact 

 thermometers the heat should be increased to 103° F. 



Temperature, third week. — The temperature should be maintained as 

 near 103° as possible up to about the eighteenth day, when it may be 

 allowed to rise to 104°.* 



Moisture and ventilation. — Oxygen is essential for the normal develop- 

 ment of the embryo chick, the amount required increasing with the growth 

 of the embryo. The eggs give off carbon dioxid, which is formed in the 

 developing chick by the combining of carbon and oxygen, and this increases 

 in amount as the embryo grows. The eggs need, therefore, only a com- 

 paratively small amount of air the first few days, as only a small amount 

 of carbon dioxid is given off at the beginning of incubation. It is asserted 

 by some authorities that the natural amount of carbon dioxid given off by 

 the eggs is beneficial, on the ground that it aids in dissolving the carbonate 

 of lime in the shells. Some experimenters go so far as to assert that it 

 is really necessary for the successful development of the embryos. The 

 writer is not yet ready to indorse this statement. 



Excessive ventilation, permitting a rapid circulation of air in the egg 

 chamber, would result in rapid evaporation of the moisture. Hence, for 

 normal development it is necessary to provide moderate, well-controlled 



♦Directions for controlling the temperature after the eighteenth day are given under the heading 

 " Hatching time." 



