408 



EMBEYOLOGY OF THE CHICK. 



smaller by the evaporation of its water. Beneath the inner 

 shell-membrane is the lohite 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 



Fig 197. — Ovum isD Structure of a Fowl's Eog, 



n, Nucleus ,• nu, nucleolus ; Sh shell ; Sli.m, 

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

 bumen) ; Y. yolk ; Yy, yellow yolk ; Wy. i, white 

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

 Gh, chalaza. (Modiiied 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, the so-called hlasloderm. 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 



