THE BIOLOGY OF THE CELL SURFACE 



properties have for cell-life, the ectoplasm by its behavior 

 as here described in the cleavage-cycle, plays a definite 

 role in the process by which the cytoplasmic mass becomes 

 two cells. 



When a cell divides, the plane of cleavage arises either 

 from within the cell-mass and extends outward, or from the 

 periphery and extends inward. The former mode, cleavage 

 by formation of the so-called cell-plate, is frequently met 

 with in plant cells. Division of animal cells takes place by 

 a furrow extending from the periphery inward. Cleavage 

 by formation of a cell-plate in animal cells is only seldom 

 found and is a modified process occurring together with 

 furrowing. The problem of cell-division for animal cells is 

 that of the origin of the furrow which separates the dividing 

 cells. The phenomena observed during the process of fur- 

 rowing in the living sea-urchin eggs by which the single 

 cell becomes two we summarize as follows: 



Currents in the cytoplasm move toward the site of the 

 future cleavage-plane; meeting there the opposing streams 

 they move inward. The ectoplasm, in whose behavior lies 

 the cause of streaming, then actively shifts by amoeboid 

 movement from the circumpolar areas toward the equator. 

 With this the egg quickly elongates. The ectoplasm moves 

 then inward along the plane pre-delineated by the cyto- 

 plasmic currents. Cleavage in a sea-urchins' egg is there- 

 fore accomplished by active movement of the ectoplasm. 



Neither cytoplasmic movements nor changes in ecto- 

 plasmic behavior are confined to animal eggs. Cytoplasmic 

 streaming has been described in plant cells, especially by 

 Ewart. Movement in the cytoplasm of Protozoa is well 

 known. Changes in shape and in activity of the ectoplasm 

 are similarly familiar from descriptions made by the earlier 

 workers on cells. Recently, investigators using the method 

 of tissue-culture have, particularly by means of the cine- 

 matograph, recorded the evanescent changes at the cell- 



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