Fig. 85. Yolk elements from the egg of the Fowl.
A. Yellow yolk. B. White yolk.
The ovum of the Fowl, at the time when it is clasped by the expanded extremity of the oviduct, is a large yellow body enclosed in a vitelline membrane. It is mainly formed of spherules of food-yolk. Of these there are two varieties; one known as yellow yolk, and the other as white. The white yolk spherules form a small mass at the centre of the ovum, which is continued to the surface by a narrow stalk, and there expands into a somewhat funnel-shaped disc, the edges of which are continued over the surface of the ovum as a delicate layer. The major part of the ovum is formed of yellow yolk. The yellow yolk consists of large delicate spheres, filled with small granules ([fig. 85] A); while the white yolk is formed of vesicles of a smaller size than the yellow yolk spheres, in which are a variable number of highly refractive bodies ([fig. 85] B).
Fig. 86. Section through the germinal disc of the ripe ovarian ovum of a Fowl while yet enclosed in its capsule.
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]).