GAMETES AND FERTILIZATION 13 



ever, lesult in the death of the embryo. It may resume its 

 development if it is brooded by the hen or artificially incubated 

 even after the egg has been kept for many days at ordinary 

 temperatures. 



The normal incubation temperature is that at which the egg 

 is maintained by the body heat from the brood-hen. This is 

 somewhat below the blood heat of the hen (io6F.). When an 

 egg is allowed to remain undisturbed the yolk rotates so that 

 the developing embryo lies uppermost. Its position is then 

 such that it gets the full benefit of the warmth of the mother. 



In incubating eggs artificially the incubators are usually 

 regulated for a heat of ioo-ioiF. (37-38C.). At this 

 temperature the chick is ready for hatching on the twenty-first 

 day. Development will go on at considerably lower tempera- 

 tures but its rate is retarded in proportion to the lowering of the 

 temperature. Below about 21 degrees Centigrade develop- 

 ment ceases altogether. 



In incubating eggs which have been cooled after laying for 

 some particular stage of the embryo which it is desired to secure, 

 three or four hours are ordinarily allowed for the egg to become 

 warmed to the point at which development begins again. For 

 example if an embryo of 24-hours incubation age is desired the 

 egg should be allowed to remain in the incubator about 27 hours. 

 Even with allowance made for the warming of the egg and with 

 exact regulation of the temperature of the incubator, the stage of 

 development attained in a given incubation time will vary 

 widely in different eggs. The factor of individual variability 

 which must always be reckoned with in developmental proces- 

 ses, undoubtedly accounts for some of the variation. The 

 different time occupied by different eggs in traversing the ovi- 

 duct, the over-night retention of eggs not ready for laying till 

 toward sundown, and especially the varying time different eggs 

 have been brooded before being removed from the nest, account 

 for further variations. The designation of the age of chicks in 

 hours of incubation is, therefore, not exact, but merely a con- 

 venient approximation of the average condition reached in 

 that incubation time. 



