224 SOME PROBLEMS OF CELL-ORGANIZATION 



theless an expression of the morphological aggregation of the proto- 

 plasmic network out of more elementary units, must, I think, be 

 accepted as a working hypothesis. Whether they are elementary 

 organisms in Altmann's sense, whether they have a persistent mor- 

 phological identity, whether they arise solely by the division of pre- 

 existing microsomes, or may undergo dissolution and reformation, 

 whether, in short, they are the self-propagating elementary bodies 

 postulated by so many eminent naturalists as the essential basis of 

 the cell, — all these are entirely open questions which the cytology 

 of the future has to solve. 



E. The Centrosome 



When we turn to the centrosome, we find clear evidence of the 

 existence of a cell-organ which, though scarcely larger than a cyto- 

 microsome, possesses specific physiological powers, assimilates, grows, 

 divides, and may persist from cell to cell, without loss of identity. 

 It is far easier to define the centrosome in physiological than in mor- 

 phological terms. In the former sense Boveri ('95, 2) defines it as a 

 single permanent cell-organ zuhich forms the dynamic centre of the cell 

 and mnltiplics by division to form the centres of the daughter-cells} 

 A centrosome is necessarily present in all cells at the time of mitosis. 

 Whether, however, it persists in the resting state of all cells is un- 

 known. The most careful search has thus far failed to reveal its 

 presence in many tissue-cells, e.g. in muscle-cells and many gland- 

 cells ; but these same cells may, under certain conditions, divide by 

 mitosis, as in regeneration or tumour-formation, and the centrosome 

 may be hidden in the nucleus, or so minute as to escape observation. 

 We must, however, remember that the centrosome often disappears 

 in the mature egg, and the same may be true of some tissue-cells. 



Van Beneden's and Boveri's independent identification of centrosome in Ascaris 

 as a permanent cell-organ ('Sy) was quickly supported by numerous observations on 

 other animals and on plants. In rapid succession the centrosome and attraction- 

 sphere were found to be present in pigment-cells of fishes (Solger. 'Sg. '90). in the 

 spermatocytes of Amphibia (Hermann, '90), in the leucocytes, endothelial cells, con- 

 nective tissue-cells and lung-epithelium of salamanders (Flemming, '91), in various 

 plant-cells (Guignard, "91). in the one-celled diatoms (Blitschli, '91), in the giant- 

 cells and other cells of bone-marrow (Heidenhain, Van Bambeke, Van der Stricht, 

 '91), in the flagellate Nottiliica (Ishikawa, '91). in the cells of marine algae (Stras- 

 burger, '92), in cartilage-cells (Van der Stricht, "92). in the cells of cancerous growths 

 (epithelioma, Lustig and Galeotti, '92), in the young germ-cells as already described, 

 and finally, in gland-cells (vom Rath, '95), and in nerve-cells (Lenhossek, '95). 

 They have not yet been found in resting muscle-cells. 



1 The fact that the centrosome is double in many cells does not conflict with this defini- 

 tion, for the doubling is obviously a precocious preparation for the ensuing division. 



