. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. [98] OVUM. Salamandrina in regard to the structure and formation of their ova. Osseous Fishes.—The ovarian ova of Os- seous Fishes, while they bear a general resem- blance to those of Vertebrata, among which they come nearest to those of Batrachia, are distinguished by several marked peculiarities. They are of middle size, and possess a strong external covering formed within the ovary. The yolk-substance contains several kinds of elements ; and the germinal vesicle is of con- siderable size. The external membrane is thic
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. [98] OVUM. Salamandrina in regard to the structure and formation of their ova. Osseous Fishes.—The ovarian ova of Os- seous Fishes, while they bear a general resem- blance to those of Vertebrata, among which they come nearest to those of Batrachia, are distinguished by several marked peculiarities. They are of middle size, and possess a strong external covering formed within the ovary. The yolk-substance contains several kinds of elements ; and the germinal vesicle is of con- siderable size. The external membrane is thick, strong, and elastic, and of a peculiar porous structure. The yolk-substance con- tains a large quantity of clear fluid, in which the albuminous granules and yolk corpuscles and the oil globules are suspended, the latter usually of large size and few in number; the germinal layer or disc is limited to a part of the yolk, varying in size from about a sixth to a half of the circumference, and the process of segmentation in this part after fecundation is consequently more limited than in Mam-. Ovum of the Gasterosteus at the time of impregnation. (-From Ransom.) A. An ovum of the Stickleback eight or ten minutes after impregnation, showing the clear re- spiratory space formed immediately upon the access of spermatozoa between the external membrane and the surface of the yolk. Towards the upper part of the figure the situation of the micropyle is indicated by the small projections in the external membrane; towards the same or upper part of the yolk the germinal disc or layer is easily distinguished from the clearer part of the yolk; and in the middle a few large coloured oil globules. B. The same ovum about three minutes after im- pregnation, showing somewhat in profile the funnel of the micropyle descending into a depression on the upper surface of the germinal part of the egg. In consequence of impregnation, however, the funnel of the micropyle has begun to rise out of the h
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