PHYSICS OF SEGMENTATION. 1 09 



a decrease in the intensities ; but in the equilibrating of poten- 

 tials the decrease in the tension is less than in the pressure, for 

 the former is already lower ; it therefore presents a relative rise. 

 This scheme offers a systematic mode of procedure for taking 

 up the examination of the various methods of producing segmen- 

 tation and development. However, since we are concerned only 

 with the bringing about of an average decrease in surface ten- 

 sion, we shall use only the second class of possibilities. 



A. Factors Decreasing Surface Tension Directly. 



One theoretically possible way of decreasing the tension di- 

 rectly is by means of either a rise in temperature or by bringing 

 to the surface (the poles of the egg) like electrical charges. 



The attracting forces, it has been seen, may be regarded as due 

 exclusively to the solvent, which for protoplasm, we have every 

 reason to believe, is only water ; consequently any direct decrease 

 in surface tension by changing the solvent chemically seems to be 

 excluded for practical reasons. But a rise in temperature lessens 

 the attraction potential directly at the same time that it increases 

 the osmotic pressure ; therewith is equilibrium done away with, 

 work being done on the egg from without. The pressure being 

 too great for the tension, the volume must increase, with absorp- 

 tion of water, until equilibrium is established. The extent of sur- 

 face therewith increases, but the potential, surface tension, decreases 

 absolutely, although, relative to the pressure opposing it, it rises 

 and the egg thus in turn does mechanical work by displacing the 

 surrounding medium. The direction of these events is thus de- 

 termined by the difference in potential. 



With this theoretically possible course of events agrees exactly 

 the observed swelling and liquefaction of cells when heated, 1 as 

 well as the starting of development in Arbacia by a rise in tempera- 

 ture, observed by Mathews. 2 This constitutes the first example 

 of factual methods agreeing with our synthetic point of view, and 

 indicates that liquefaction consists either in increasing the pres- 

 sure by analytic processes, or in absorption of water, or in 

 both. 



1 Greeley, A. W., BIOLOGICAL BULLETIN, IV., 3, 1903; V., i, 1903. 



2 Ainer. Jour. Physiology, IV., VII. 



