IO4 E. G. SPAULDING. 



within a system in relation. to a fall in a third potential outside of 

 the system ; at the same time the extensity factors of the first 

 two may diminish. Under these circumstances work is done on 

 the system. Conversely, two potentials within a system may 

 get equilibrium and, at the same time, both decrease absolutely 

 in relation to a third external, which rises. The system then 

 does work. Both of its extensity factors may however increase 

 in this process, but this does not necessarily mean a gain in 

 energy, for energy equals potential x extensity. 



Concrete instances of these possible cases, which have been 

 selected as bearing directly on our special problem, we shall find 

 in the physical events making up the process of segmentation 

 and cleavage. 



3. SURFACE TENSION AND OSMOTIC PRESSURE. 



In the instance of the normal progress of the event of segmen- 

 tation, i. e., with a cleavage of both nucleus and cytoplasm, no 

 matter whether this is the result of natural or artificial fertiliza- 

 tion, it is an undeniable fact that, coincident with and as a culmi- 

 nation of all the processes taking place in the cell there is a de- 

 crease in surface tension in at least certain parts of the surface. 

 This follows necessarily from the change in the radius of curva- 

 ture of the approximately spherical form of the egg to the 

 increased radii of certain parts of the surface of the constricted 

 form. For surface tension, the potential of surface energy, varies 

 inversely with the radius of curvature, and is greatest in the 

 spherical form. From this it follows that in cleavage there is in 

 any case an average decrease in surface tension accompanying 

 the redistribution of this. The very fact that these changes 

 occur may be advanced as a proof that in protoplasm we are 

 dealing with either a solution or a fluid. Accordingly the sur- 

 face energy is to be considered as due to or identical with the 

 "attracting forces " (cohesion) of the fluid particles. This redis- 

 tribution of surface tension may be correlated with and is doubt- 

 less confirmed by observed protoplasmic streamings. 1 



It would not, however, occur of itself; it must have an ultimate 

 cause, either within or without the system (the egg), and in either 



1 Cf. Butschli, "Protoplasm." 



