36 GENERAL SKETCH OF THE CELL 



E. 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 gener- 

 ally regarded as the especial organ of cell-division, and in this sense 

 as the dynaviic centre of the cell (Van Beneden, Boveri).^ 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 cell- 

 division and the fertilization of the Qgg. As a rule it lies out- 

 side, though near, the nucleus, in the cyto-reticulum, surrounded 

 by a granular, reticular, or radiating area of the latter known 

 as the attraction-spJiere or centrosphere (Figs. 5, 6, 7).- 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 anticipa- 

 tion 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 nucleus.^ 



1 The centrosome was discovered by Van Beneden in the cells of Dycyemids ('76), and 

 first carefully described by him in the egg of Ascaris seven years later. The name is due 

 to P.overi ('88, 2, p. 68). ' 



2 Cf. p. 229. 



^ Its nature is more fully discussed at p. 224. 



