Page:Proceedings of the Royal Society of London Vol 60.djvu/214

194 oogonial mitoses, is intranuclear, and it is often separated from the well-defined persistent nuclear wall by a clear space. The chromosomes, when assembled on the spindle, at the equator, are seen to be twice as numerous as in the oogonial nuclei, seen in profile we counted them as twenty in number. We were unable to distinguish any such grouping of the chromosomes as would lead to the conclusion that the chromosomes of tbe male and female nuclei respectively had so far preserved their original identity as to appear in the form of two separate groups. The long interval of time which, in , elapses between fertilisation and the first nuclear division possibly may admit of a more thorough mingling or fusion of the parental chromosomes than would seem to be the case in some animals, e.g., the Copepoda as described by Riickert and by Hacker.

During the diaster stage the connecting achromatic fibres are at first very distinct, but they soon become fainter, and no cell-plate is formed across them. The two daughter nuclei gradually pass into the state of rest, each being first hemispherical, with crenate projections on the flattened side turned towards its sister nucleus. Only after nuclear division is complete does the first cell wall appear. The cell is sometimes spherical when this happens, and then it is divided into two similar hemispheres. Further divisions may then appear, whilst the general contour of the embryo still remains more or less spherical. These cases occurred most frequently when the germinating spores were illuminated on all sides. But most commonly the first cell wall cuts the spore into two dissimilar halves, one of which grows out and forms a rhizoid. Often this projection is already apparent even before the first nuclear division occurs, and in any case one of the two daughter nuclei always passes down into the protuberance.

The immediately succeeding divisions have been sufficiently described by Thuret and others, but we may remark that the division of the nuclei in all cases precedes the formation of a cell plate, which is not formed in connexion with the achromatic connecting fibrils as in the higher plants.

The doubled number of the chromosomes is retained daring the vegetative divisions of the thallus, and is constant throughout the somatic cells of the mature Fucus plant. Hence it follows that the reduction in the number of the chromosomes (in the female plants), .is associated with the differentiation of the oogonium—the mother cell of the sexual products. Thus Fucus, in this respect, approximates more closely to the type of animal oogenesis than to that which obtains in those higher plants in which the details of chromosome reduction has been followed out.

Regarded from the standpoint of the number of its chromosomes, the Fucus plant resembles the sporophyte of the higher plants, whilst