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J. GRAY 



greater than | of the diameter of the rings (Text-fig 6, b), the 

 form of the drop changes in the same way as that of a dividing 

 egg. The further A is moved away from B the more convex 

 do the ends of the cyhnder become, and the more marked is the 

 development of the ' cleavage ' furrow ; finally the drop is 

 resolved into two separate parts. 



There is, however, one respect in which the protoplasmic 

 system differs from that of an oil-drop. When a drop of oil 

 is divided into two — as in Plateau's experiment — it is a simple 

 matter to reverse the process and reform a single drop of oil. 



Text-fig. 6. 



Form of completely divided egg in (a) normal sea-water. (6) Acid 



sea-water. 



In the case of the living cell this does not occur. It would 

 seem that this is due to the existence of a surface laj^er (Traube 

 membrane) which is automatically formed when protoplasm 

 comes into contact with water, and that the blastomeres fail 

 to fuse with each other just as oil-drops fail to fuse together 

 if they are shaken in smaller drops in the presence of a soap 

 or any similar substance which can form a condensation layer 

 at the surface of the oil. 



The conclusion reached is that division of the cell is brought 

 about by the elongation of one axis of the cell, and that the 

 cleavage fm'row^ results as an equilibrium between this process 

 and the normal surface tension at the cell-surface. It need 



