EGOS AND INCUBATION 493 



in a small way; but, in the larger commercial trade, eggs are 

 candled over sets of electric lights arranged for this purpose. 

 The incubation of the egg of the hen occupies a period 

 of 21 days. The following are some of the more important 

 changes that take place during incubation. During the first 

 twenty-four hours the blastoderm enlarges to about a half 

 inch in size, within which the first stages of head and some 

 other parts appear. During the second day the heart begins 

 to beat and the blood to flow. By the end of the third day 

 the veins and arteries are considerably developed, and the 

 young chick turns on its left side. On the fourth day the 

 wing folds, and the folds forming the legs appear. The beak 



Figure 236. — The egg. Left egg dead germ; center, fertile egg on 7th day; 

 right egg, infertile. Reproduced from Circular 99 of the California 

 Agricultural Experiment Station. 



begins to form on the eighth day, and shows its horny shape 

 on the twelfth. The entire shell except the air cell is occu- 

 pied by the chick by the twelfth day. The feathers appear 

 first on the eighth day, and by the thirteenth cover the body 

 to the length of one fourth inch. At this time the nails of 

 the feet appear. On the fourteenth day the chick changes 

 its position and extends lengthwise, the beak reaching the 

 inner shell membrane. The air cell has been gradually in- 

 creasing in size, and by this time is much larger. From now 

 on, the chick increases in development to the twenty-first 

 day. The following interesting description of the hatching 

 process is given by Professor Lewis ;* 



* Poultry Laboratory Guide, 1910. 



