CHEMICAL MECHANICS OF CELL-DIVISION 467 



one another and tend to close up the cleft again. If they fail to do so 

 it can only be because the molecular attractions have been weakened, 

 i. e., the surface-tensionTiiminished. In a rubber balloon there is no 

 such attraction across the cleft; the tension is purely transverse and is 

 not exerted perpendicularly to the surface as it is in a fluid, and so 

 there is nothing to prevent a cleft from extending deeper and deeper 

 into the equator of the rubber balloon provided the tension of the 

 encircling band is reduced thereby to a greater extent than the tension 

 of the balloon is increased. 



In 1895 it was suggested by Loeb that phenomena of Protoplasmic 

 Streaming are what really lead to cell-division. He pointed out that 

 in cell-division the protoplasm streams from the equator of the cell 

 in opposite directions toward the two nuclei; the violence of these 

 streaming movements, he suggested, brings about the mechanical 

 separation of the two cells. 



The streaming of protoplasm from the equator toward the poles 

 suggests that the phenomenon antecedent to cell-division is a diminu- 

 tion of surface-tension in the equatorial region and not, as Biitschli 

 suggests an increase. That such equatorial diminution in surface- 

 tension will bring about the division of droplets into two is very readily 

 shown by means of the following simple experiment : 



The formation of Soaps at the surface of oil-droplets, results as we 

 have seen in Chapter XIII, in a diminution of the surface-tension of the 

 droplets; if the formation of soap is local, that part of the surface upon 

 which the soap is formed tends to spread. Since commercial olive oil 

 almost invariably contains traces of Fatty Acid the result of bringing an 

 alkali in contact with a drop of such oil will be the formation of soap at 

 the points of contact. If, now, a drop of Olive Oil which is not too 

 large (about 2 to 3 mm. in diameter) be floated on a layer of water, and 

 a thread saturated with tenth normal alkali (NaOH or K0H) be brought 

 gently into contact with a diameter of the drop, the almost immediate 

 effect is the division of the drop into two. The phenomena accompany- 

 ing this division are perfectly characteristic. Instantly the edges of 

 the drop (the ends of the diameter along which the thread lies) recede 

 from the thread, forming a notch at each end of the diameter, and 

 violent streaming-motions occur at the surface away from the thread 

 and toward the opposite poles of the drop. These streaming move- 

 ments may be so violent as to rotate the droplets into which the drop 

 divides through as much as 360. If the division does not occur too 

 rapidly the streaming may result in the two droplets being connected 

 by a thread of oil, which may be central or to one side, and it may then 

 be clearly seen that the mechanism which brings about the snapping of 

 this thread is the violent streaming in opposite directions which takes 

 place in the drops. Phenomena almost exactly resembling those 

 described by Loeb in dividing ova may readily be observed (Fig. 30). 

 Frequently, also, processes resembling Pseudopodia are thrown out by 

 the droplets in the act of their division. 



The segmentation of the drop is not due to mechanical division by 



