96 THE POPULAR SCIENCE MONTHLY. 



colored eggs, and those which possess concealed or covered nests, 

 white eggs. 



But few of those who break the shells of the cooked eggs of 

 our common domestic fowls at the breakfast-table ever think of 

 the wonderful nature of the structure they crush, or of the com- 

 plex chemical nature of the contents consumed as food. The 

 white, fragile cortex called the shell, composed of mineral matter, 

 is not the tight, compact covering which it appears to be, for it is 

 everywhere perforated with a multitude of holes. Under the mi- 

 croscope the shell appears like a sieve, or it more closely resembles 

 the white perforated paper sold by stationers. The shell of the 

 egg is lined upon its interior everywhere with a very thin but 

 pretty tough membrane, which, dividing at or very near the ob- 

 tuse end, forms a small bag which is filled with air. In new-laid 

 eggs this follicle appears very small, but it becomes larger when 

 the egg is kept. In breaking an egg this membrane is removed 

 with the shell, to which it adheres, and therefore is regarded as a 

 part of it, which it is not. The shell proper is made up mostly of 

 earthy materials. The proportions vary according to the food of 

 the bird, but ninety to ninety-seven per cent is carbonate of lime. 

 The remainder is composed of two to five per cent of animal mat- 

 ter, and one to five of phosphate of lime and magnesia. 



If a farmer has a flock of one hundred hens they produce in egg- 

 shells about one hundred and thirty-seven pounds of chalk annu- 

 ally ; and yet not a pound of the substance, or perhaps not even an 

 ounce, exists around the farm-house within the circuit of their 

 feeding-grounds. The materials of the manufacture are found in 

 the food consumed, and in the sand, pebble-stones, brick-dust, 

 bits of bones, etc., which hens and other birds are continually 

 picking from the earth. The instinct is keen for these apparently 

 innutritious and refractory substances, and they are devoured 

 with as eager a relish as the cereal grains or insects. If hens are 

 confined to barns or out-buildings, it is obvious that the egg- 

 producing machinery can not be kept long in action unless the 

 materials for the shell are supplied in ample abundance. 



Within the shell the animal portion of the egg is found, which 

 consists of a viscous colorless liquid called albumen, or the white, 

 and a yellow globular mass called the vitellus, or yolk. The 

 white of the egg consists of two parts, each of which is enveloped 

 in distinct membranes. The outer bag of albumen, next the shell, 

 is quite a thin, watery body, while the next, which invests the yolk, 

 is heavy and thick. But few housekeepers who break eggs ever 

 distinguish between the two whites, or know of their existence 

 even. Each has its appropriate office to fulfill during the process 

 of incubation or hatching, and one acts, in the mysterious process, 

 as important a part as the other. 



