CENTROSOME. The centrosome was long thought to be a permanent organ of all animal and plant cells. It is an extremely small body, usually less than 1/25000 of an inch in diameter, and is almost always surrounded by a system of radiating threads. It was first described by the zoologist Flemming in 1875, but was named and brought into prominence by another zoologist, Boveri, in 1888. Three years later a French botanist claimed to have found centrosomes in the lily. The general appearance and behavior of the centrosome in animal cells may be illustrated by the first division of an egg (Fig. 1).
An examination of the figure will indicate that the centrosome plays an essential role in the division of the nucleus, since it gives rise to the spindle which moves the chromosomes to their place in the new nuclei. On account of this function, it was claimed that the centro some must necessarily be present in all cells. However, it proved impossible to demonstrate a centrosome in some resting cells, like muscle cells, and so it was concluded that, in some cases, the centrosome might not be a permanent organ, but, rather, that it must be formed anew at each successive cell division. In the early nineties this view was quite generally accepted. While a centrosome was described and figured in great detail in the lily and various botanists began to find centrosomes in various groups of plants, other botanists could not demonstrate it even in the lily. In 1896 a vigorous investiga tion by several botanists of different nationali ties, but chiefly American, working under the
.direction of Strasburger at Bonn practically solved the centrosome problem in plants. A tentrosome behaving as in animal cells was found in two of the alga and two of the fungi (Figs. 2 and 3).
On the other hand, it was definitely estab lished that in the Scouring Rush, Equisetum, no centrosomes are present during nuclear and cell division. It was also demonstrated that no centrosomes are present in the Gymnosperms some being involved in the process. At the two divisions in spore mother cells a weft of fibres appears, closely surrounding the entire FIG. 3.
nucleus; the weft becomes draivti into many poles, which are geaduidlY drawn together into two opposite groups, so that the familiar bi polar spindle is established (Fig. 4).
In ordinary vegetative divisions, the spindle first appears as pair of dome-shaped prom inences, or caps, at opposite poles of the nucleus. No centrosome is concerned. Since that time many centrosomes have been demon strated in various alga and fungi; but in plants higher than these no centrosomes have been proved, except as the centrosome may appear as the blepharoplast (q.v.) during spermato genesis. Consult Wilson, E. B., The Cell in Development and Inheritance); also various articles in Jahrbitcher fiir wissenschaftliche Botanik (1896 and 1897).