_a._ Connective-tissue capsule of the ovum; _b._ epithelium of the capsule, at the surface of which nearest the ovum lies the vitelline membrane; _c._ granular material of the germinal disc, which becomes converted into the blastoderm. (This is not very well represented in the woodcut. In sections which have been hardened in chromic acid it consists of fine granules.) _w.y._ white yolk, which passes insensibly into the fine granular material of the disc; _x._ germinal vesicle enclosed in a distinct membrane, but shrivelled up; _y._ space originally completely filled up by the germinal vesicle, before the latter was shrivelled up.] In addition to the yolk there is present in the ovum a small protoplasmic region, containing the remains of the germinal vesicle, which forms the germinal disc (fig. 86). It overlies the funnel-shaped disc of white yolk, into which it is continued without any marked line of demarcation. It contains numerous minute spherules of the same nature as the smallest white yolk spherules. Impregnation takes place at the upper extremity of the oviduct. In its passage outwards the ovum gradually receives its accessory coverings in the form of albumen, shell-membrane, and shell (fig. 87). [FIG. 87. DIAGRAMMATIC SECTION OF AN UNINCUBATED FOWL'S EGG. (Modified from Allen Thomson.) _bl._ blastoderm; _w.y._ white yolk. This consists of a central flask-shaped mass and a number of layers concentrically arranged around it. _y.y._ yellow yolk; _v.t._ vitelline membrane; _x._ layer of more fluid albumen immediately surrounding the yolk; _w._ albumen consisting of alternate denser and more fluid layers; _ch.l._ chalaza; _a.ch._ air-chamber at the broad end of the egg. This chamber is merely a space left between the two layers of the shell-membrane. _i.s.m._ internal layer of shell-membrane; _s.m._ external layer of shell-membrane; _s._ shell.] [FIG 88. SURFACE VIEWS OF THE EARLY STAGES OF THE SEGMENTATION IN A FOWL'S EGG. (After Coste.) _a._ edge of germinal disc; _b._ vertical furrow; _c._ small central segment; _d._ larger peripheral segment.] [FIG. 89. SURFACE VIEW OF THE GERMINAL DISC OF FOWL'S EGG DURING A LATE STAGE OF THE SEGMENTATION. _c._ small central segmentation spheres; _b._ larger segments outside these; _a._ large, imperfectly circumscribed, marginal segments; _e._ margin of germinal disc.]