STUDIES IN ARTIFICIAL PARTHENOGENESIS. 



165 



upon the egg colloids. The actual evidence in support of this 

 statement will be given at a later point (p. 196). 



In the first paper of this series, after showing that all sub- 

 stances which produce membrane elevation cause a lowering 

 of surface tension, I proposed a theory of membrane-elevation 

 based on a simple consideration of the forces in equilibrium 

 at the vitelline membrane. At the time I was not yet aware 

 of the semipermeable nature of this membrane, and hence in 

 discussing the various forces acting upon it, I did not consider 

 osmotic forces as being involved. The theory therefore re- 

 quires slight modification in the light of this new fact. Let 



FIG. i. 



us suppose in Fig. I that the shaded area is the vitelline mem- 

 brane. The arrows represent the forces acting upon it. Arrow 

 A directed outward indicates the outward force due to the 

 osmotic pressure of the dissolved substances within the membrane. 

 Were gels present in the egg we should also have to add the 

 swelling pressure of these. Directed inward are two arrows, 

 arrow B is the force due to the osmotic pressure of the salts 

 outside the membrane, arrow C is the radial component of the 

 surface tension of the membrane. As the membrane is slightly 

 thicker than twice the range of molecular action, it is really 

 composed of two surfaces of surface tension, i. e., an outer 

 surface of contact with the sea-water and an inner surface of 



