Nature (journal)/Volume 1/Number 9/Zoology

In a note, published last February, on the development of the egg in those curious crustacean parasites, the Sacculinae, M. Gerbe stated that the ovules of these animals are formed at first of two transparent vesicles or cells, each furnished with a nucleus and a common membrane; that one of these cells enlarges considerably, and that at the maturity of the ovum, the large cell in which the elements of the vitellus have been developed predominates to such an extent that the smaller one forms only a minute prominence at one pule of the ovum. M. Gerbe compared the large cell to the yolk in the eggs of birds, and regarded the small one as representing the germ or cicatricula. This homology was also applied by him to the ova of the Arachnida and Myriopoda.

M. E. van Beneden finds that the ovules are not at first composed of two closely applied cells, but that they form a single cell, formed of a transparent protoplasm, containing a few strongly refractive globules, and of a vesicular nucleus with a nucleolus. With these are observed others of an elongated form, ​and possessing two nuclei, but showing no signs of division, and others, again, which present at one end a small bud, the size of which increases until it equals that of the mother-cell, when one of the nuclei passes into the daughter-cell, and as the division between the two cells becomes more distinct, the appearance described by M. Gerbe is produced. M. van Beneden could could not, however, detect any cell-membrane.

The development of the ovule then goes on much as described by M. Gerbe, one of the daughter-cells being enlarged much more rapidly than the other, and acquiring a vitelline character. When it has attained a diameter of 0.015–0.018 millimetre, a cell-membrane (vitelline membrane) may be detected, which, however, only covers the larger cell. With these ova others are found in which the smaller or polar cell is no longer to be distinguished, but which present at one point a depression representing the surface to which it was attached; the ova when deposited never present the least trace of the polar-cell; but after oviposition the ovaries contain numerous cells, resembling the original mother-cells, which are really the polar-cells thrown off from the mature ova. These, M. van Beneden believes, become the mother-cells of a new set of ovules. He supports this opinion by several instances derived from Crustacea of other groups, such as Caligus, Calvella, Lernanthropus, Congericola, Anchorella, Lernaeopoda, &c., in which analogous phenomena occur.

M. van Beneden remarks, that in the ova of Sacculina segmentation of the whole contents of the ovum takes place, and he describes the process. This, as he says, excludes the idea of a cicatricula, which occurs only where a great part of the nutritive material exists outside the protoplasm of the ovicell, as in birds. Hence there can be no comparison between the egg of the Sacculinae and that of birds, nor has it any special analogy to that of the Arachnida and Myriopoda.