LIVING SUBSTANCE 



69 



recently, however, as if, in addition to the two previously known 

 general constituents, the protoplasm and the nucleus, a third 

 exists, the polar corpuscle, central corpuscle, or centrosome. 



The centrosome (Fig. 10) has become known in detail only very 

 recently. It had, indeed, been noticed when the peculiar 

 phenomena of nuclear division in cell-multiplication were in- 

 vestigated twenty years ago ; but not until later was it recog- 

 nised by van Beneden ('83, '87) and Boveri ('87, '88, '90) as 

 an important element in the cell, which reproduces like the 

 nucleus in the increase of cells by division, van Beneden came 

 to believe that the centrosome, like the nucleus and the proto- 

 plasm, is a general cell-constituent. This idea was supported by 



FIG. 10. a, Pigment-cell from the pike ; the centrosome with its protoplasmic radiation lies be- 

 tween the two nuclei. (After Solger.) b, Leucocyte from the larva of a salamander; the 

 centrosome with the aster lies at the right of the dumb-bell-shaped nucleus. (After 

 Flemming.) c, Egg-cell in the act of dividing; there is a distinct protoplasmic radiation 

 about each of the two centrosomes. (After Boveri.) 



the observations of Flemming, Solger, Heidenhain, and others, 

 who found one or more centrosomes in other kinds of cells, such 

 as leucocytes, pigment-cells, epithelium-cells, etc., and even when 

 they were not undergoing division. Nevertheless, in a great 

 number of cells it has not been possible up to the present time 

 to demonstrate such a body. Perhaps this is due to its nature. 

 It is a granule that is very difficult to find in protoplasm on account 

 of its minuteness, and no structure whatever has been proved in 

 it by the help of the microscope. Moreover, as a rule it is not 

 stained by the usual staining-reagents. The endeavours of M. 

 Heidenhain to find for it specific staining-media, such as exist for 

 the nucleus, have not yet led to wholly satisfactory results. Its 

 presence is clearly revealed by the protoplasmic radiations by 

 which in certain conditions of the cell it is surrounded. In the 

 division of the cell the protoplasm arranges itself around the 



