356 Sir W. H. Flower on the General 



found in most text-books, with the following description 

 (mainly condensed from Gadow) : — 

 The newly-laid egg of a bird consists of — 



A. The Yolk, in the germinal disk of which [a) the deve- 

 lopment of the chick begins. The yolk is yellow and opaque, 

 consists mainly of granular globules of albuminous and 

 fatty matter, and is enclosed in the vitelline membrane (V). 



B. The White, or Albumen, transparent and colourless, 

 but showing a definite structure, some parts being denser 

 and firmer than others, notably the chalazce (6), twisted cord- 

 like structures, which pass between the poles of the egg and 

 the yolk, serving to keep the latter in position. 



C. The thin but tough and fibrous Shell-membrane, con- 

 sisting of two layers, which tend to separate at the broad 

 end of the egg, and develop an air-chamber (c) between 

 them. This chamber does not exist in perfectly fresh eggs, 

 but makes its apjjcarance and increases in size as the white 

 of tlie egg loses in bulk from evaporation. 



D. The Shell, which is the last part forraedj being deposited 

 upon the surface of the shell-membrane by the glandular 

 walls of the uterine dilatation of the oviduct. The shell 

 consists of an organic basis or matrix, hardened with calca- 

 reous matter (mainly calcium carbonate, with a small quan- 

 tity of magnesium carbonate and calcium phosphate), and 

 generally consists of three layers : — 1, the internal or mam- 

 raillary layer, the roughened projections or mammilla? of 

 which rest upon the shell-membrane ; 2, the porous layer, 

 constituting the chief thickness of the shell and permeated 

 by numerous vertical canals, simple in most birds, but 

 branched in Ratitae (except Apteryx) ; 3, the thin outer or 

 cuticular layer, apparently structureless ; either very poor in 

 calcium salts (and in this case smooth and glossy), or con- 

 siderably infiltrated with calcareous matter, and so giving the 

 rough chalky appearance seen in some eggs. This extends 

 over the apertures of the canals of the poi'ous layer, but air 

 readily passes through it when dry, though not so when moist. 

 In the eggs of some birds this layer appears to be absent. 

 The colour of the shell is produced by pigment deposited on 

 it during its passage through the oviduct ; sometimes it is 



