IO6 E. G. SPAULDING. 



result of action between the egg and its environment, in which 

 case the egg may either have done work, as, e.g., in a medium 

 of less osmotic pressure, or work may have been done upon it 

 by means of mechanical agitation, heating, abstraction of water, 

 or fertilization by a spermatozoon. Any of these possible events 

 must take place in complete agreement with the general conditions 

 above outlined, and will in every case of cleavage ultimately 

 condition a change in the relation of two kinds of energy, surface 

 and osmotic. 



That this is true is shown by consideration in detail of the 

 characteristics of these two energies. Surface energy is due to 

 the mutual attraction of the molecules of a fluid, which here forms 

 one part, viz., the solvent, of protoplasm. The molecules at a 

 certain distance from the surface are each free to adjust their 

 mean position under the influence of the surrounding molecules, 

 the mean position being that in which each is acted upon equally 

 on all sides, with the result that the mutual attraction is not 

 rendered manifest. At the surface, however, if this be free, or 

 relatively free, as is shown by experiment (/. e., if a chemical dif- 

 ference between medium and egg exists) the molecules are vir- 

 tually acted upon only by those lying internal to them. The 

 result is a system of forces manifesting their action throughout 

 the fluid and at right angles to the surface, /. t\, radially and 

 tending to reduce the surface to the least possible area, i. e., the 

 spherical form. The surface in contact with a chemically different 

 medium so that mixture does not take place, acts like or is in 

 fact a membrane or film. 



The factors by which variations in the attraction of these 

 particles for each other are determined are : (i) Their chemical 

 nature, and consequently (2) the density of the fluid, (3) the 

 temperature, a rise in which decreases the tension, (4) the pres- 

 ence of electrical charges. 



Osmotic (volume) energy is identical with the mutually re- 

 pelling forces of molecules in a solvent, and follows the law of 

 gases. Its two factors are accordingly volume and pressure, and 

 in a natural event as the former increases the latter decreases. 

 The conditions upon which variations in this depend are accord- 

 ingly (i) for equal weights of dissolved substance, and equal 



