408 



EMBRYOLOGY OF THE CHICK. 



smaller by the evaporation of its water. Beneath the inner 

 shell-membrane is the white or albumen ( W) of the egg. This 

 white is a mixture of proteids, fats, extractives, and saline 

 matters. In this layer again we can distinguish between a 

 fresh and a stale egg: in a stale egg the outer albumen is 

 always more or less soft and watery, whereas in a fresh egg 

 it is quite firm. This white contains no less than 86 per cent 

 of water. The yolk lies within the white, and is enclosed in 



?K.T)». 



Fig. 197. — Ovum and Strdctube of a Fowl's Eqg. 



n. Nucleus » uu, nucleolus ; Sh. shell ; Sfe,m, 

 shell membranes ; Ac, air chamber ; W, -white (al- 

 bumen) ; F, yolk ; Fy, yellow yolk ; Wy. i, white 

 yolk of flask ; Wy. ii, other layers of white yolk ; 

 C/b, chalaza, (Modified after Allen Thomson.) 



a thin membrane called the vitelline membrane, which is partly 

 elastic in nature, and easily seen puckered up when we break 

 the yolk of an egg. 



The yolk (Y) itself is of two kinds, yellow and white; it, 

 however, mainly appears yellow in colour, except at one place 

 where a small round pale area exists, about the sixth of an 

 inch across, tlie so-called blastoderm. The yellow yolk is sur- 

 rounded externally by a thin layer of white yolk which passes 

 under the blastoderm, where it extends into the middle of the 



