THE BIOLOGY OF THE CELL SURFACE 



down of the germinal vesicle; but before this break-down 

 come the intense ectoplasmic changes incident to fertiliza- 

 tion. In an egg like that of Chaetopterus after the germinal 

 vesicle breaks down the spindle goes to the metaphase and 

 comes to lie at the periphery before fertilization; although 

 this translocation of the spindle indicates streaming in the 

 cytoplasm, this soon diminishes; most intense streaming is 

 resumed after fertilization. The cytoplasm of the unfer- 

 tilized sea-urchin egg when ripe for fertilization is as fluid 

 as any that I know, the germinal vesicle having broken 

 down some time before in the ovary, but cytoplasmic 

 streaming of measurable degree is demonstrable only after 

 fertilization. Moreover, this cytoplasmic streaming is 

 rhythmical: during cleavage the tides ebb and flow. And 

 always ectoplasmic activity heralds their flood. It may 

 very well be that in all cells ectoplasmic behavior as a 

 response to changes in the environment initiates cytoplas- 

 mic streaming. 



In eggs so far used in the study of the currents in cyto- 

 plasm the two streaming movements from poles to equator 

 by opposing each other seem to bring about cell-division. 

 Theories concerning their cause have not been wanting, the 

 chief of which is that the currents are due to surface-tension. 



The theory that the division of the cell-body is caused 

 by changes in surface-tension is upheld by many investiga- 

 tors. Some maintain that the cell divides because of 

 increased surface-tension over the spindle-poles, whereas 

 just as many workers are certain that the increase is at 

 the equator. In many quarters what is considered the 

 strongest proof that the division of the cell is due to changes 

 in surface-tension comes not from observations and experi- 

 ments on the cells themselves but from study of oil-drops 

 suspended in water or solutions. It is a curious fact that 

 here too the theorists are not agreed: some contend that an 

 oil-drop divides because of increased surface-tension at the 



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