THE CENTROSOME No element of the cell has aroused a wider interest of late than the remarkable body known as the centrosome, which is now generally regarded as the especial organ of cell-division, and in this sense as the dynamic centre of the cell (Van Beneden, In its simplest form the centrosome is a body of extreme minuteness, often indeed scarce larger than a microsome, which nevertheless exerts an extraordinary influence on the cytoplasmic network during celldivision and the fertilization of the egg. As a rule it lies outside, though near, the nucleus, in the cyto-reticulum, surrounded by a granular, reticular, or radiating area of the latter known as the or centrosphere (Figs. 5, 6, It may, however, lie within the nuclear membrane in the linin-network (Fig. 107). In some cases the centrosome is a single body which divides into two as the cell prepares for division. More commonly, it becomes double during the later phases of cell-division, in anticipation of the succeeding division, the two centrosomes thus formed lying passively within the attraction-sphere during the ordinary life of the cell. They only become active as the cell prepares for the ensuing division, when they diverge from one another, and each becomes the centre of one of the astral systems referred to at p. 49. Each of the daughter-cells receives one of the centrosomes, which meanwhile again divide into two. The centrosome seems, therefore, to be in some cases a permanent cell-organ, like the nucleus, being handed on by division from one cell to another. There are, however, some cells, e.g. muscle-cells, most gland-cells, and many unicellular organisms, in which no centrosome has thus far been discovered in the resting-cell ; but it is uncertain whether the centrosome is really absent in such cases, for it may be hidden in the nucleus, or too small to be distinguished from other bodies in the cytoplasm. There is, however, good reason to believe that it degenerates and disappears in the mature eggs of many animals, and this may likewise occur in other cells. At present, therefore, we are not able to say whether the centrosome is of equal constancy with the 1 The centrosome was discovered by Van Beneden in the cells of Dycyemids ('76), and first carefully described by him in the egg of Asearis seven years later. The name is due to Boveri ('88, 2, p. 68).
2 Cf. p. 229.
Its nature is more fully discussed at p. 224.