farr: cytokinesis of pollen-mother-cells 263 



avoid the attempt to harmonize the cell-plate in plants with 

 any equatorial differentiation in animal cells; and it seems ad- 

 visable, at least, to use the term " zwischenkorper " or "diasteme" 

 for such structures in animals, reserving the word "cell-plate" 

 for the higher plants only. 



e. Theories of cell-division 



Very numerous theories of cell-division and related phenomena 

 have been offered. Perhaps most students have believed that it 

 is an expression of the working of the architectural mechanism of 

 the cell. One school believes that there is a radiating system of 

 organic rays about the centrosome. To this belong: Van Beneden, 

 Boveri, Flemming, Hertwig, Solger, Zimmermann, Heidenhain, 

 and Kostanecki. Von Fick (27) takes exception to this in that 

 it does not take account of the surface relations; and Rhumbler 

 assumes that cell-division is brought about by a combination of 

 surface forces and those of the internal mechanism. 



A number of investigators have attempted to place the study on 

 a physico-chemical basis by proposing what have been referred to 

 as the dynamic theories of cell-division. Gallardo (22) reviews 

 the literature up to 1902. Fol in 1873 suggested that the achro- 

 matic figure resembles the arrangement of iron filings over a 

 magnet. Soon afterwards Strasburger referred to the resemblance 

 of the spindle to a magnetic field as a peculiar coincidence. A 

 number of workers modeled fields of force to represent mitotic 

 figures in various kinds of media; Giard in 1876 in liquids; Faraday 

 with sulphate of quinine crystals in spirits of turpentine; Gueblard 

 with copper and lead acetates; and Biitschli with gelatin. Errera 

 in 1880 discredited the belief that there are electrical phenomena 

 at work in the cell by experiments with cells of Tradescantia 

 dividing in the field of a powerful magnet. He interpreted cell- 

 division as a hydrodynamic process. Fick in 1897 made the sug- 

 gestion that macroscopic models cannot reproduce microscopic 

 phenomena, since capillarity and other molecular forces are at 

 work in the one but not in the other. Prenant contends that 

 there are two forces: a traction or centripetal force; and a com- 

 pression or a centrifugal force. Meves discredits the idea of the 

 fibers as representing lines of force on the ground that they often 

 cross; but Wilson afiirmed that "the crossing of rays is not neces- 



