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Rh separate them. Although the position of the vesicle in the middle of the yelk-cell affords evidence rather in favour of its being regarded as a young cell, yet it is not altogether inconsistent with its character as a nucleus; for it is only during the earliest formation of the cell that the nucleus is required to be connected with it; it is frequently disconnected at a later period, and lies loose in the cell. At that stage of development, however, in which the vitelline-membrane closely encompasses the germ-vesicle, it is impossible to decide whether it lie in the middle or on the wall of the cell. This point, therefore, is of more ideal than practical importance for the prosecution of the investigation.

The third point relates to the signification which attaches to the individual parts of the germ-vesicle. It may be hollow consistently with both views. Although we are not as yet acquainted with any hollow nuclei in plants, we have nevertheless found nuclei in cartilages which were hollow, and decidedly to be regarded as cytoblasts. The question now arises, what are Wagner’s spots or spot? If the germ-vesicle be considered to be a young cell, one of them may be its nucleus, and the rest cell-contents, or nuclei of young cells, which will be developed afterwards; if it be regarded as nucleus, the spots may either be nucleoli, or merely its contents. It is a fact in favour of the former view, that only one spot is present in most instances, the others being usually produced at a later period. Wagner has sometimes observed one or more minute points in this single spot, and has delineated them from Alcedo hispida, Lepus cuniculus, Ovis aries, &c.; I have also sometimes met with small points of this kind which gave the spot, in some degree, the appearance of a nucleus adhering to the wall of the cell, and containing within it these little points as its nucleoli. Meanwhile, their presence is too inconstant, and they are generally too indefinite, to permit of our attributing any importance to them in the decision of the present question. The extraordinary number in which they frequently occur is opposed to their being regarded as nucleoli within the germ-vesicle, presuming it to be a cell-nucleus, for in fishes they sometimes fill the entire vesicle, at least, being closely crowded, they cover