The Dynamics of a Model of Cell Division. 127 



allowed to flow onto opposite poles of the drop at the same time 

 and rate. The drop quickly elongates toward the pipettes, i. e., 

 toward the poles, and constricts along the equator, and some- 

 times divides into two. The smaller the drop, the more certain 

 the division, provided the operator has sufficient skill. 



The alkali forms soap which reduces the surface tension on the 

 polar areas, and the hydrostatic pressure within the drop causes 

 these areas to bulge, whereas the relatively higher surface tension 

 of the equatorial region causes it to constrict until a barrel-shaped 

 figure is formed, which rapidly becomes hour-glass shaped. The 

 equatorial surface film contracts and the polar surface film spreads, 

 causing vortex movements. The enlargement of the polar fields 

 spreads the soap over larger areas, and the area of unaltered 

 surface tension is reduced to a narrow equatorial band. This 

 band, being partially released by reduction of tension at its edges, 

 acts as a sphincter and constricts until it cuts the oil drop into 

 two. This constriction of the oil drop may be considered as a 

 rough model of cell division. 



T. B. Robertson in a recent paper 1 claims that exactly the 

 opposite changes take place in cell division. He divided the oil 

 dropp by lacing on it a linen thread 0.4 mm. in diameter, previously 

 soaked in the alkali. If the drop is not more than 1/10 c.c. in 

 volume the thread cuts it in two. This is due to gravitation of 

 the thread, the alkali merely lessening the resistance to the cutting. 

 I found that better results were obtained by adding a little alkali 

 to the water instead of soaking the thread in it. 



The various points in Robertson's argument cannot be con- 

 sidered here, and the reader is referred to his paper. The most 

 striking fallacy is that in Fig. 2, p. 699, Mi and M% are not the same 

 distance below the curved line alsmd. 



1 Arch. f. Entwicklungsmech., 1913, XXXV, p. 692. 



