REPRODUCTION. 193 



egg. The structure of the hen's egg is more complicated. 



The outside shell consists of earthy matter (lime) depos- 



ited in a net-work of animal matter. 



It is minutely porous, to allow the 



passage of vapor and air to and fro. 



Lining the shell is a double mem- 



brane (membrana putaminis} resem- 



bling delicate tissue-paper. At the 



larger end^it separates to enclose a FlG . 161 ._ Theoi . etical Egg , 



bubble of air for the use of the chick. or Cell: *> viteiime mem- 



_ T in braue ; y, oleaginous pole ; 



JNext comes the albumen, or "white, o, albuminous pole; p, 



in spirally arranged layers, within 

 which floats the yolk. The yolk is s ermiDal . dot - 

 prevented from moving towards either end of the egg by 

 two twisted cords of albumen, called chalazce / yet is al- 

 lowed to rise towards one side, the yolk being lighter than 

 the albumen. The yolk is composed of oily granules 

 (about ir ^ r of an inch in diameter), enclosed in a sac, called 

 the vitelline membrane, and disposed in concentric layers, 

 like a set of vases placed one within the other. That part 

 of the yolk which extends from the centre to a white 



Fio. 162. Longitudinal Section of Hen's Egg before incubation: a, yolk, showing 

 concentric layers ; o', its semi-fluid centre, consisting of a white granular sub- 

 stance the whole yolk is enclosed in the vitelline membrane ; b, inner dens* 

 part of the albumen ; &', outer, thinner part ; c, the chalazse, or albumen, twisted 

 by the revolutions of the yolk ; d, double shell-membrane, split at the large end 

 to form the chamber,/; e, the shell ; h, the white spot, or cicatricula. 



13 



