Memoirs: On the Histology of the Ovary and of the Ovarian Ova in certain Marine Fishes

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Abstract
In fishes which have pelagic ova and an annual spawning season, the formation of yolk in the developing ova to be shed at a given spawning season commences some months after the close of the preceding spawning season. The active development of the annual crop of ova does not take much more than six months. The formation of yolk always commences near the surface of the cytoplasm and extends inwards. In those eggs which develop separate oil globules, a few of these of small size are present long before the formation of yolk commences. The eggs of the mackerel form an exception to this statement. In immature specimens of sole, turbot, brill, &c., examined during the spawning season, the largest ova in the ovaries are found to contain scattered oil globules, and these are also present in the largest transparent ova in spent ovaries of these species. When the formation of yolk takes place in such eggs the oil globules form a zone internal to that of the yolk. The essential peculiarity of the spent ovary is the presence in it of the ruptured follicles, from which the ripe eggs have escaped. The follicular epithelium in these appears to disintegrate and dissolve. The cavity is obliterated by the contraction of the follicle, which forms a mass of cells and fibres, and is finally absorbed soon after the commencement of the formation of yolk in the eggs for the following season. In the spent ovary there are a number of eggs which have not reached the ripe condition, which die, and are not discharged from their follicles, but absorbed in situ. In the fresh state they are visible as opaque amorphous masses. Similar opaque masses are also seen in immature ovaries in which spawning has never occurred. Here also they are aborted dead ova, which are undergoing disintegration and absorption. They are scattered singly in the ovarian tissue, and their development is arrested at an early stage, before the formation of yolk has made any progress, if even it has commenced. In those in which death has only recently occurred the germinal vesicle is seen to be shrunken, and to contain a single large spherical nucleolus. The vitelline nucleus is first seen as a stained corpuscle in contact with the germinal vesicle. I have not been able to follow the mitosis of the germ-cells, or to trace back the vitelline nucleus, but consider it most probable that the latter is identical with the centrosome which remains in the ovum after the last division of the germ-cell. In ova of plaice and flounder the vitelline nucleus separates from the germinal vesicle, moves towards the surface of the ovum, and is afterwards found at the inner border of the yolk-layer. It becomes surrounded with yolk, and ceases to be visible. In Syngnathus acus there are often two or even more vitelline nuclei in a single ovum. These are probably produced by the division of a single body. If the vitelline nucleus is the centrosome, its disappearance forms an interruption to the persistence of the centrosome as an extra-nuclear body, since the directive spindle is provided with new centrosomes. The germinal vesicle in the Teleostean ova examined consists at first of a single large nucleolus, a nuclear network, and a surrounding membrane. I could not resolve the network into a continuous filament, or into separate chromosomes. At the next stage the vesicle is larger, and there are several nucleoli in contact with the nuclear membrane. In still larger ova the nucleoli are still at the periphery, but there is a central region in the vesicle distinguished by the presence of separate feathery fibrils, the centrosomes of Rückert. After the formation of yolk has commenced the membrane of the vesicle is wrinkled. The nucleoli migrate from the periphery of the vesicle, and are found around and among the central fibrils. There are indications in the ova of the turbot that the substance of the nucleoli is absorbed into the central fibrils to form the chromosomes of the polar mitoses, but the actual formation of these chromosomes was not followed.

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