I50 SCIENCE PROGRESS. 



othcTwise astonishing differences observed between nearly 

 allied plants are, on this hypothesis, no longer surprising. 



It seems to me that a consideration of an exceptional case 

 like the one just quoted, is especially useful in enabling us 

 to form a judgment as to how far the whole process of cell 

 division may be the result of physical forces operating in the 

 cell, or how far we are justified in invoking the aid of a special 

 organ of division to rescue us from intellectual uncertainty. 

 It must be remembered that, if we have recourse to the 

 centrosome as the prime operator in these matters, we 

 practically give up attempts at arriving at a solution of the 

 difficulties. For we are no nearer to comprehending the 

 phenomena of cell division than we should be as regards 

 the origin of protoplasm itself if we were to assume that 

 it was brought here in the first instance on an aerolite. 

 On the other hand, if we accept the view of Heidenhein 

 that the centrosome is a mere insertion point of the forces 

 set free in the protoplasm, its nature becomes a secondary 

 matter, and we know, as a matter of fact, that its position 

 may vary, it may be intranuclear during the spindle forma- 

 tion, as in the 2iniz'alens variety of Ascaris viegalocephala, 

 or, on the other hand, it may be extranuclear, as in the other 

 [bivalens) variety of the same animal. These questions as to 

 position, however, lose their importance if we take this view 

 of its role in karyokinesis. Furthermore, the disparity of size 

 between the two centrosomes belonging to opposite ends of 

 a dividing nucleus, which was pointed out amongst others 

 by Reinke,^ no longer astonishes us, nor does the fact 

 that here and there spindle fibres seem occasionally to mis- 

 take an isolated granule for a centrosome, judging at least 

 from the manner in which they diverge towards it. Still 

 once more, the facts noticed by Belajeft " and others, that 

 during the division of the pollen mother cells of certain 

 flowering plants the spindle fibres are frequently at first not 

 directed to two points, but only later converge thither, be- 

 come less unintelligible ; the directions of the strains have, 



' Reinke, "Zellstudien," Theil II., Archiv f. Mikr. Anat., Bd. xlix. 

 -Belajeff, " Zur Kenntniss d. Karyokinese b. d. Pflanzen,'' Flora, 

 Erg.-Bd., 1894. 



