BIRDS' EGGS 171 



the students of birds' eggs with being the first to appreciate 

 the intimate relationship between gulls and plovers 

 which was afterwards proved up to the hilt on anatomical 

 lines by Huxley and others. 



What is roughly called the yolk of a bird's egg is the 

 true egg-cell, which has been enormously dilated by its 

 store of reserve material. A drop of genuine living-matter 

 lies like an inverted watch-glass on the top of the yolk, 

 and from this drop the whole of the embryo is formed, 

 the yolk being simply a nutritive legacy. All round the 

 yolk there is a delicate vitelline envelope, and outside this 

 lies the white of egg or albumin, secreted by the walls of 

 the oviduct, and also nutritive. Round about the white of 

 egg there is a delicate, tissue-paper-like double shell-mem- 

 brane, the two layers of which are separated at the broad 

 end of the shell to form an air-chamber. Outside the shell- 

 membrane is the porous shell, which is formed from the 

 walls of the oviduct. It is often stained with pigments, 

 and it consists mostly of carbonate of lime, with a small 

 proportion of carbonate of magnesia and phosphate of 

 lime and magnesia. Experiments have shown that a hen 

 can make a normal carbonate of lime shell, although 

 there is no carbonate, but only other salts of lime in its 

 food. 



One of the first facts that strikes us in connection with 

 birds' eggs is that there are so few in a clutch, compared 

 with the number in lower animals. The wren may have 

 eight or nine eggs, the mallard a dozen, and the long-tailed 

 tit as many as twenty ; but these large numbers are quite 

 exceptional. Many birds, such as the golden eagle and the 

 black guillemot, usually lay only two ; while others, like the 

 common guillemot and the little auk, have come down to 

 one. There are two ways of looking at the smallness of 

 the family. On the one hand, birds are typically flying 



