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 (tneinbrana putaminis) resem- 

 bling delicate tissue-paper. At the 

 larger end, it separates to enclose a FlG . 161 . _^7 tical E , g , 

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



braue ; y, oleaginous pole ; 



Next comes the albumen, or " white, a, albuminous poic ; p, 



n j -, .,, . Purkinjean, or germinal, 



in spirally arranged layers, within vesicle ; >, wagnerian, or 

 which floats the yolk. The yolk is germiua1 ' dou 

 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 -5--^- of an inch in diameter), enclosed in a sac, called 

 the viteiiine 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; a', its semi-fluid centre, consisting of a white granular sub- 

 stance the whole yolk is enclosed in the viteTline meinbrane; ft, inner dense 

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

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

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



13 



