. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Diagrammatic representation of the three principal kinds of ovarian ova, in sections (tlie relation, of size not, however, being maintained). A. Mammiferous ovarian ovum ; z. zona pellucida or vitelline membrane of some authors; v. granular yolk substance; g. germinal vesicle, with its nucleus or germinal spot; A', young ovum of a molluscous animal. B. Young ovarian ovum of a bird; v. m. vitelline membrane; v. large yolk corpuscles of two kinds— viz. those of the yellow external part, and those of the cavity; g. ge
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Diagrammatic representation of the three principal kinds of ovarian ova, in sections (tlie relation, of size not, however, being maintained). A. Mammiferous ovarian ovum ; z. zona pellucida or vitelline membrane of some authors; v. granular yolk substance; g. germinal vesicle, with its nucleus or germinal spot; A', young ovum of a molluscous animal. B. Young ovarian ovum of a bird; v. m. vitelline membrane; v. large yolk corpuscles of two kinds— viz. those of the yellow external part, and those of the cavity; g. germinal vesicle without spot or nucleus, but granular diffused maculae; v. d. small granular yolk substance forming the vitelline disc. c. Ovarian ovum of a batrachian reptile; v. m. vitelline membrane; v. d. vitelline disc, or forma- tive yolk-substance deeply coloured, and surrounding the large granular yolk substance, in one nearly entirely, in the other, c', only half round the ovum ; a. germinal vesicle. two kinds of organised particles, — viz., of smaller granules nearly similar to those which form the whole yolk in the last group, and of larger cells, usually non-nucleated, and fat vesi- cles, which constitute the greater part of the mass. The first or granular part of the yolk constitutes a thin disc, limited to one region of the surface, — viz., the upper side in the vicinity of the place occupied by the germinal vesicle, while the cellular substance of the yolk forms the larger spherical mass of the egg. Besides these two, there are also various intermediate forms, which seem to be stages of transition between the other kinds of struc- lural elements. The germinal vesicle in the ripe ovarian ovulum, is situated in the centre of the granular disc, and after its disappear- ance, the process of segmentation is limited to that disc. The germinal or blastodermic mem- brane, or layer of cells, extends, therefore,"at first, no farther than this granular disc, and
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