The protoplasm of a living cell con.sists of a semifluid granular substance, called the cytoplasm, one or more nuclei, and sometimes centrosomes and plastids.
The cytoplasm is largely concerned in the formation of spindle fibres and centrosomes, and such structures as the cell membrane, cilia, or flagella, the coenocentrum, nematoplasl~ or vibrioids and physodes are also products of its activity.
In the higher plants the structures which have been often described as centrosomes are too indefinite in their constitution.
The centrosomes in.
The centrosomes which play so important a part in cell division may be found either lying within or at one side of the nucleus in the vegetative condition of the cell.
Centrosomes may be single, but usually two are lying close together in the attraction-sphere.
After the division and cleavage of the chromosomes of the original nucleus have taken place they pass from the equator to the poles of the spindle, rearranging themselves close to the separated centrosomes to form daughter nuclei.
In many pathological cells undergoing indirect segmentation, centrosomes appear to be absent, or at any rate do not manifest themselves at the poles of the achromatic spindle.
In the course of division two bodies appear in the cytoplasm, and behave as centrosomes during the karyokinesis; they gradually become threadlike and coil round each daughter nucleus.
It was at one time thought that the centrosomes played an important part in the fertilization of plants, but recent researches seem to indicate that this is not so.