GENERAL ACCOUNT. 223 



has fertilisation been effected, but the egg has already been 

 developing for a period which varies in different cases, but 

 amounts on an average to about eighteen hours. 



When the egg is laid, development stops. To set it going 

 again, to start development afresh, all that is necessary is that 

 the egg should be kept at a temperature about equal to the 

 blood-heat of the parent bird. This is normally effected by 

 incubation, the hen sitting on the egg, and so keeping it 

 warm ; but it may be effected equally well by artificial means. 

 A certain amount of moisture, and free access of air, are neces- 

 sary to insure normal development. The rate of development 



FIG. 98. The yolk of a Hen's Egg at the thirty-sixth hour from the com- 

 mencement of incubation. The structure of the embryo at this stage is 

 shown on a larger scale in Figs. Ill and 112. x H. 



AD. area pellucida of the blastoderm. AK, area opaoa. AV, area vasculosa. 

 EM, embryo. SM, vitelline membrane. Y, yolk-sac. 



varies to a slight extent according to the season of the year, 

 autumn eggs developing more slowly than spring eggs ; or 

 according to the temperature, if an artificial incubator is em- 

 ployed. The length of time the egg takes to travel down the 

 oviduct, during the whole of which time it is developing, varies 

 considerably, and individual variations may occur from other 

 causes ; but, as a rule, the chick hatches on the twenty-first 

 day from the commencement of incubation. The age of an 

 embryo is always calculated from the commencement of incuba- 

 tion, or from the time of placing the egg in the incubator ; to 

 obtain the true age there must be added to this the time 

 during which the egg was developing, in its passage down the 



