238 



J. GRAY 



place (at 15° C.) about fifty minutes after fertilization, and 

 takes roughly three minutes. The process of cleavage is shown 

 diagrammatically below. 



It is important to note that during cleavage there is a progres- 

 sive increase in the length of the main axis of the egg ; this is 

 just as distinctive as the production of the shorter axis which 

 is brought about by the development of the cleavage furrow. 



Text-fig. 3. 



Stages in normal cell-division of egg of Echinus miliaris. 



As the egg elongates so the polar regions become more and 

 more convex, while the equatorial region becomes more and 

 more concave. 



In making a comparison between the dividing egg and an 

 oil/water system it is convenient to consider the fusion of 

 two oil-drops rather than the division of a single drop into two 

 equal parts. When two oil-drops fuse it is obvious (Text-fig. 4) 

 that a reversal of the process would approximate very closely 

 to the process of cell- division. 



Now, in fusing together the amount of free energy at the 

 surface of the oil is reduced in the ratio of 5 : 4, so that when 

 the single drop is mechanically shaken into two equal parts, 



