farr: cytokinesis of pollen-mother-cells 261 



front of the furrow. In the case of Synchytrium, Harper (28c) 

 suggests that there are two processes involved in cleavage. "If 

 in Synchytrium the nuclei are centers for the formation of kino- 

 plasm, and it proceeds outward from them by diffusion in all 

 directions till it reaches the plasma membrane; this will be corre- 

 spondingly increased in thickness, and if the mass be decreasing 

 in volume by loss of water and tending to split up like a mass of 

 drying starch, the membrane might perhaps press into the furrows 

 thus formed, so as to become the surface layer of the forming 

 segments." 



In the Myxomycetes (28/), Harper finds further evidence of the 

 correlation of loss of water with cell-division. In Didymium cell- 

 division is accompanied by an extrusion of cell-sap, "which is 

 initiated and controlled by changes in the spore-plasm." The 

 writer suggests that "If we add to this conception of the nucleus 

 as a center of water retention the further conception, suggested 

 by its relation to cell-plate formation in the higher plants and the 

 ascus, that it is a center for the production of plasma membrane 

 materials, we have two factors which would work in harmony to 

 bring about the process of progressive cleavage described, since 

 the diffusion outward from the nucleus of substances to be used 

 in forming the plasma membrane would again tend to cause the 

 cleavage planes to pass midway between any given pair of nuclei." 

 In his work on Fuligo (28d) the same author found that while 

 the cleavage furrows cut midway between two nuclei, the process 

 has no relation to the orientation of the preceding nuclear division 

 figures. 



d. Cytokinesis in animals 



In the animal kingdom there is considerable variation in the 

 process of cytokinesis, though it hardly displays as wide a range 

 of types as the plant kingdom. In the typical case the cell be- 

 comes ellipsoidal and there is a tendency for the two halves, each 

 containing one nucleus and one centrosome, to round up, which 

 is accomplished by a constriction furrow. This is particularly 

 true of some of the small eggs, as those of worms. On the other 

 hand, some eggs with large yolk, as those of the frog, retain their 

 general spherical form, and the furrow is more in the nature of a 

 line of dehiscence. Terni (71) has recently published figures which 

 show that amphibian cells may divide either by a broad curved 



