[Illustration: FIG. 73. Diagrammatic longitudinal section of a hen'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). LECTURE XV THE PROCESS OF FERTILIZATION Cell-division and nuclear division--The chromatin as the material basis of inheritance--The rôle of the centrosphere in the mechanism of division--The Chromosomes--Fertilization of the egg of the sea-urchin according to Hertwig--Of the egg of Ascaris according to Van Beneden--The directive divisions, or the extrusion of the polar bodies--Halving of the number of chromosomes--The same in the sperm-cell--Reducing division in parthenogenetic eggs--In the bee--Exceptional and artificial parthenogenesis--Rôle of the centrosphere in fertilization and in parthenogenesis. NOW that we have made ourselves acquainted with the two kinds of germ-cells on the union of which 'sexual reproduction' depends, we may proceed to a more detailed discussion of the process of fertilization itself. But it is indispensable that we should take account of the processes of nuclear and cell-division, as these have been gradually recognized and understood in the course of the last decade. It may appear strange that the processes of division should throw light on the apparently opposite processes of cell-union, but it is the case, and no understanding of the latter is possible without a knowledge of the former.