. Chambers's encyclopaedia; a dictionary of universal knowledge. ails of this sexual attraction,and for the importance of sexual selection in evolv-ing decorations and charms, the reader may bereferred to Darwins Descent of Man, where thesubject is very fully discussed. Birds associatetogether to very ditterent degrees; thus, eagleslive separately, the two sexes only coming to-gether at breeding time ; the doves live in pairs ;many birds are similarly monogamous, but livein society, except during the breeding-season, whilefowls and some running birds are polygamous, onemale ruling over a numbe
. Chambers's encyclopaedia; a dictionary of universal knowledge. ails of this sexual attraction,and for the importance of sexual selection in evolv-ing decorations and charms, the reader may bereferred to Darwins Descent of Man, where thesubject is very fully discussed. Birds associatetogether to very ditterent degrees; thus, eagleslive separately, the two sexes only coming to-gether at breeding time ; the doves live in pairs ;many birds are similarly monogamous, but livein society, except during the breeding-season, whilefowls and some running birds are polygamous, onemale ruling over a number of females. See Sex. Eggs.—The size of the egg is very generally pro-portionate to that of the parent, but there are manystriking exceptions. The very centre of the eggcontains what is knoA\Ti as \vhite yolk which risesas a plug to the surface. This white spot, usuallylying upi^ermost when the egg is opened, is thearea (hlustoderm) where by the multiplication andarrangements of cells the embryo chick is mappedout. The yellow yolk is traversed by concentric. Fig. 13.—Section of Towls Egg :a, central pluK of white yolk; h, position of germinal area;c, yellow yolk ; d, wliite of egg ; e, tread or chalaza ; /, air-space between outer membranes. layers of white, which also forms a thin layerround the outside. As tlie egg passes down theupper portion of the oviduct it becomes coated with fluid white of egg or albumen, which is separatedfrom the yolk by a (vitelline) membrane inclosingthe latter. The Huid albumen is also disposed inlayers, separated by tibrovis networks, and almostevery one is familiar with two thick sets of fibres(known as the tread) which extend from each endof the yolk into the surrounding allmmen. In thelower portion of the oviduct (the uterus) the eggbecomes, as we have already noted, inclosed in alimy shell. This is an organic structure and is two-layered. Its texture and microscopic charactersdiffer in the various orders, and appear to be tnilydiagnostic.
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