CHAPTER IX 



Incubation 



To the casual observer, an egg consists roughly 

 of three parts, but to the scientific investigator 

 these are capable of several subdivisions. The 

 shell, composed of lime, forms a protection ; but it 

 is not an impenetrable cover. It is very porous. 

 It has between the particles of lime an innumer- 

 able number of very small holes, which allow the 

 air to pass freely backward and forward during 

 the process of incubation. Next is the white, the 

 albumen. This is not all of one character; one 

 portion is much denser than the other. The watery 

 portion is placed around the outer surface next to 

 the shell. 



In the interior is the yolk, which in itself is, as a 

 whole, lighter in density than the white, therefore 

 its tendency is to come to rest upon the surface of 

 the white. But the yolk is also differently con- 

 stituted, one portion being a little heavier than an- 

 other, with the consequence that the heavier por- 

 tion moves downward and the lighter up. There 

 is a good deal of misunderstanding about the very 

 dense jellylike portions of white. Popular con- 

 ception says the young chick is developed from 

 them, but this is wrong. They simpl}^ consist of 

 denser and more gelatinous albumen, and have ac- 

 quired that twisted, corkscrew appearance and shape 

 by the revolutions of the yolk in traveling down the 

 ovary of the hen. But this twisting assists in keep- 



139 



