CHARACTERS AND HABITS OF BIRDS 21 



geese will often begin to incubate after having laid about the 

 number of eggs that they could cover. Many fowls will do the 

 same, but most fowls lay for several months before attempting 

 to incubate, and in many races not more than two or three per 

 cent of the hens ever incubate. 



Development of the embryo in a bird's egg. The condition 

 required to produce a live bird from a fertile egg is the continu- 

 ous application of a temperature of about 102 or 103 degrees 



FIG. 8. Embryo (after seven days' FIG. 9. Chick ready to break 



incubation) . shell 



Fahrenheit from the time the heat is first applied until the em- 

 bryo is fully developed and ready to emerge from the shell. In 

 nature the heat is applied by contact with the bodies of the 

 parent birds. Development of life will start in an egg at about 

 10 degrees below the temperature required to maintain it, but 

 at this temperature the germ soon dies. The temperature in 

 incubation may occasionally go higher than 103 degrees or may 

 be as low as 70 degrees for a short time without injury to the 

 germ. Some germs will stand greater extremes of temperature 

 than others, just as some living creatures will. 



The first stages of the development of life in the egg of a 

 bird may be observed by holding the eggs before a strong light 

 in a darkened room. White-shelled eggs are the best for this 



