Fig. 73.Diagrammatic longitudinal section of ahen's egg before incubation, after Allen Thomson.Bl, germinal disk.GD, yellow yolk.WD, white yolk.DM, vitelline membrane.EW, albumen.Ch, chalaza.S, shell membrane.KS, shell.LR, air chamber.
Fig. 73.Diagrammatic longitudinal section of ahen's egg before incubation, after Allen Thomson.Bl, germinal disk.GD, yellow yolk.WD, white yolk.DM, vitelline membrane.EW, albumen.Ch, chalaza.S, shell membrane.KS, shell.LR, air chamber.
We now understand why the eggs of many animals should be of such enormous size and often of such complex structure. The eggs of birds are especially remarkable in this respect, and it has till recently been disputed whether they are really morphologically equivalent to a single cell. But this is undoubtedly the case, and though only the small thin germinal disk (Fig. 73,Bl) with its nucleus is the active part of this cell—the cell-body proper—yet all the rest—the enormous sphere of yolk with its regular layers of yellow (GD) and white (WD) yolk, the concentric layers of fluid albumen (EW) round about this, the chalazæ (Ch), and finally, the delicate shell membrane (S) and the limy shell (KS)—belong to this cell, and have arisen in connexion with it (Fig. 73).