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Show 1886.] OVUM OF LEPIDOSIREN. 277 membrane of the germinal vesicle is not always very obvious; but as there is a sharp contrast between the protoplasm of the germinal vesicle and the surrounding substance of the ovum (indicated by the much deeper staining of the former by borax carmine), there comes to be an appearance of a delicate membrane surrounding the germinal vesicle ; this membrane has not a double contour. In younger ova there is a very distinct membrane bounding the germinal vesicle externally ; this membrane is more easily to be distinguished in m y preparations from the fact that it is very deeply stained. Although the boundaries of the germinal vesicle are distinct enough, there is no cavity dividing it from the egg-protoplasm ; the granules of the latter are everywhere in contact with the germinal vesicle. In several preparations the germinal vesicle had shrunk ; but in every case observed by me the wall of the germinal vesicle was covered with egg-protoplasm granules, thus showing that the spaces surrounding the germinal vesicle in that figure are not natural, but due to alterations caused by the contraction of the germinal vesicle. The interior of the germinal vesicle is occupied by a finely-granular matter, which is almost homogeneous throughout. In certain tracts, however, the granules are more deeply stained and often larger; these granules form a reticulum (Plate X X I X . fig. 3). In the ovum of Triton, Iwakawa illustrates (loc. cit. pi. xxiv. fig. 27) a very similar condition of the nucleoplasm. The germinal spots are very numerous, and form a layer surrounding the germinal vesicle; the latter is shown on a superficial view in fig. 3, and in transverse section in Plate X X I X . fig. 4. The germinal spots are of very varying size, and usually oval or circular in form ; borax carmine stains them more deeply than the surrounding nucleoplasm ; they are evidently not homogeneous, but appear to consist of an outer sheath of stout consistency, and very deeply stained by the reagent, and within this an apparently more fluid core which is not deeply stained. The presence of numerous germinal spots is figured by Ayers for both Ceratodus (plate xviii. fig. 76) and Lepidosiren (plate xvii. fig. 27). It is also characteristic of the Teleostean ovum \ and is mentioned by Messrs. Balfour and Parker 2 in Lepidosteus. Among Amphibia Triton shows the same condition of the germinal spots. In ova of the stage represented in Plate XXVIII. fig. 2, the substance of the ovum which immediately surrounds the germinal vesicle differs from the rest in being more loosely compacted. In ova which are distinguished by the enormously increased functional activity of the follicular epithelium, the germinal vesicle could not be found. In all probability, however, m y failure to find the germinal vesicle is not due to its absence. 1 Brock, " Beitrage z. Anatomie und Histologie der Geschlechtsorgane der Knoohenfische," Morphol. Jahrb. Bd. iv. (1878) p. 505, pi. xxviii. figs. 8, 11. 2 " O n the Anatomy and Development of Lepidosteus" Phil. Trans. 1884. |
