The Energy of Segmentation 309 



solution, but, rather, to this plus the pressure of the surface fihn or 

 membrane. Accordingly, the numerical value of this must be 

 found, that it may be known whether it is significant for our com- 

 putation or not. 



The formula by which this pressure due to the tension of the 

 surface,'^ if this be only a film like the surface of a drop of water, 

 may be computed, is 



2 t 

 ^ r 



in which t is the coefficient of surface tension and r the radius of a 

 sphere. This / is determined from the capillary action of a fluid 

 in accordance with the formula, t = \ g r h D (g = action of 

 gravity, r = radius of tube, h = height to which the fluid is drawn 

 up, D = the density). 



Pfefi^er^^ gives this coefficient as .01 g. cm. in relation to that 

 of water as unity. Since other determinations are lacking, I 

 made use of this, although, of course, it must be admitted that 

 this coefficient might vary greatly with different kinds of proto- 

 plasm. 



Substituting this value in the formula, 



2 t 



r 



p = .0055 atmos. pressure 

 For the two-cell stage, with each cell an oblate ellipsoid, the form- 

 ula is more complicated: here 



area 

 (a = long axis, c = short) 



"The best treatment of the general problem of surface tension, etc.,which I have found is M. Heiden- 

 hain's Die allgemeine Ableitung der Oberfliichenkrafte, etc., in Anatomische Hefte, erste Abteilung, 

 vol. xxvi. Wiesbaden. 1904. 



-" Plasmahaut u. Vakuolen, Abhandl. d. Math.-phys. Kl. d. Sachs. Ak. d. Wis., 16, 185 (1891); 

 cited by Hober, Physikalische Chemie der Zelle u. Gewebe, s. 38; Leipzig, 1902. This is the only 

 determination I have been able to find. 



*For this formula I am indebted to Dr. C. R. Maclnnes, of Princeton University. 



