THE REPRODUCTIVE GLANDS. 423 



other hand, can readily be obtained and have been studied in greater 

 detail. In the following pages we shall confine our attention to 

 the composition of birds' eggs, which is best understood. The egg 

 proper is here surrounded by the so-called white of egg, which in 

 turn is enclosed in a double membrane, and is covered by the shell. 

 These additional structures, however, are not formed in the ovary, 

 but are produced during the passage of the egg through the oviduct 

 from material, which is here secreted by the lining cells. 



The Shell. The shell consists essentially of an organic matrix of 

 the character of keratin, which is largely impregnated with lime 

 salts. Of these, calcium carbonate is the most abundant, and con- 

 stitutes about 90 per cent, of the weight of the entire shell. In 

 addition, we find a small amount of magnesium carbonate, as also 

 phosphates of both elements. Water is present to the extent of 

 only about 1 per cent. The pigments met with in birds 7 eggs 

 are closely related to the biliary pigments, and, like these, are 

 derived from the common pigment of blood. The oorhodein, which 

 presents a reddish or brownish-red color, is supposedly identical 

 witii hsematoporphyrin ; while the blue or green pigment, which is 

 termed oocyanin, is composed partly of biliverdin, and is in part a 

 blue derivative of bilirubin. 



The membranes of birds 7 eggs consist essentially of keratin, but 

 contain also a small amount of mineral salts, of which calcium 

 phosphate is the most abundant. 



In fishes and amphibia the egg envelope is represented by a trans- 

 parent gelatinous material, which seems to consist almost exclusively 

 of mucin. In the invertebrates chitin and skeletins take the place 

 of the keratin of birds 7 eggs, but in some the latter also is found. 



The weight of the shell and membranes in the case of hens 7 eggs 

 represents about 9 to 11 per cent, of the total weight of the egg, 

 while the albumen constitutes about 60.5 per cent, and the yolk, 

 viz., the ovum proper, the remaining 29 per cent. The total weight 

 of hens 7 eggs may vary between 40 and 70 grammes. 



The Albumen. The albumen or white of egg, as obtained directly 

 from the raw egg, appears as a faintly yellow, exceedingly viscid, 

 semiliquid material. On microscopical examination this can be 

 shown to consist of compartments, which are limited by very 

 delicate membranes, and enclose the albumen proper. These 

 membranes are continuous with the so-called chalazse and the 

 membranes immediately beneath the shell, and are, like these, 

 composed of keratin. 



The albumen proper may be separated from its membranous con- 

 stituents by pressing the material through a cloth, and then appears 

 as an opalescent fluid, which is only slightly viscid, and can be 

 filtered without much difficulty. Its reaction is distinctly alkaline, 

 and the specific gravity about 1 .045. On boiling, it coagulates to a 

 compact mass, which in the case of hens 7 eggs is entirely opaque. 

 In some birds, however, such as the swallow, the crow, the finch, 



