926 EMBRYOLOGY. 



The shell is very solid, and impregnated with calcareous salts ; it is 

 perforated by a multitude of cavities opening on its surface. When 

 closely examined, it is found to have a more or less shining aspect, and in 

 its mass fine sand-like particles. 



Landois, who has completed the researches of Wittich on the ovum of 

 birds, distinguishes several layers in the shell ; otherwise their number 

 varies with the species. The shell, acting merely as a protecting covering, 

 is all the more solid and complicated as the eggs are more exposed to 

 the inclemency of the weather. 



Independently of the testaceous membrane which Landois attaches to 

 the shell as a fibrous layer, this authority also recognises the uterine gland 

 layer and the spongy layer. The former is much impregnated with cal- 

 careous salts, and deeply studded with little round bodies which are the 

 remains of the uterine glands, and give to the shell its sandy appearance. 

 The spongy layer is structureless, and analogous to solidified mucus. 



This description demonstrates that the ovum of birds is distinguished by 

 the considerable volume of its vitellus, and the addition to it of those 

 accessory parts which give the egg its large dimensions and solidity. 

 These differences will readily be understood, when it is remembered that the 

 embryo must find in it all the materials necessary for its development, 

 which takes place external to the parent. It is from the vitellus and the 

 albumen that the young creature derives the elements which the mammal 

 finds in the uterine mucous membrane to which it is fixed. 



