10 Farmers' Bulletin H67. 



INCUBATION PERIOD IN BIRDS. 



In birds the incubation period corresponds to the gestation period 

 in mammals. The period which elapses from the time eggs are put 

 on to hatch until the chicks emerge is shown below. Young birds are 

 nourished in the embryonic stages of their development by absorbing 

 from the yolk the nutriment needed. 



Incubation period of various birds. 



Days. 



Goose 30 



Duck 28 



Turkey 28 



Guinea fowl 25 



Chicken 21 



Pigeon 17 



Much may be learned by any one of a studious nature by careful 

 observations of the deA r elopment of the embryo of chickens. The 

 germinal disk may be plainly seen even in a fresh-laid egg, which, 

 whether fertile or not, looks the same to the naked eye. When put 

 into an incubator, however, the fertile egg begins to develop rapidly, 

 and the transformation in the brief period of three weeks is won- 

 derful. The course of this development in fertile eggs can be dis- 

 tinguished easily from day to day by the familiar process of candling 

 eggs, or it can be studied in detail by carefully breaking the eggs 

 into a saucer. 



DETERMINATION OF SEX. 



As the growth of the embryo progresses, sex becomes apparent. 

 Sex seems to be determined by one of the chromosomes, and is prob- 

 ably settled at the time of conception. 4 The determination of sex 

 is therefore a matter of chance over which the breeder probably has 

 no control. For practical purposes, the breeder will find that, over 

 a period of years, he gets nearly equal numbers of both sexes. 



FECUNDITY. 



The subject of fecundity is of great practical importance to 

 breeders of animals. Fecundity ^depends very largely on the number 

 of eggs which are matured at a given time by the female. Animals 

 in which multiple births are the rule (of which sows are a common 

 example) not only mature a considerable number of egg cells at 

 one period of heat, but each one. is fertilized by a separate sperm 

 cell. In some cases, how r ever, the fertilized egg may divide and 

 twins or a larger number of young result. Such twins are known 

 as identical twins, and are usually of the same sex, with very strong 



*See note (2) page 6. 



