The elements of embryology (1874) The elements of embryology elementsofembryo01fost Year: 1874 I.] THE OVARIAN OVUM. 19 of which forms the bed on which the blastoderm rests. The whole yolk is invested with the vitelline membrane, this again with the white; and the whole is covered with two shell-membranes and a shell. 11. Such an egg has however undergone most important changes while still within the body of the hen ; and in order to understand the nature of the structures which have just been described, it will be necessary to trace briefly the history of the egg from the stage when it exist
The elements of embryology (1874) The elements of embryology elementsofembryo01fost Year: 1874 I.] THE OVARIAN OVUM. 19 of which forms the bed on which the blastoderm rests. The whole yolk is invested with the vitelline membrane, this again with the white; and the whole is covered with two shell-membranes and a shell. 11. Such an egg has however undergone most important changes while still within the body of the hen ; and in order to understand the nature of the structures which have just been described, it will be necessary to trace briefly the history of the egg from the stage when it exists as a so-called ripe ovarian ovum in the ovary of a hen up to the time when it is laid. If one of the largest capsules of the ovary of a hen which is laying regularly be opened, it will be found to contain a nearly spherical (or more correctly, ellipsoidal with but slightly unequal axes) yellow body enclosed in a delicate membrane. This is the ovarian ovum or esf2f. Examiued with care the ovum, which is tolerably uniform in appearance, will be found to be marked at one spot (generally lacing the stalk of the capsule and forming the pole of the shorter axis of the ovum) by a small disc differing in appearance from the rest of the ovum. This disc is known as the germinal disc or discus proligerus. It consists of a lenticular mass of Fig. 4, Section through the Germinal Disc of the ripe Ovarian Ovuji op 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 by the action of th
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