ARTIFICIAL PARTHENOGENESIS 487 



stances makes manifest the mechanism oC tlie change brought 

 about by their agency, and its similarity to that caused by butyric 

 acid or other fatty acid. The former all lead to the abnormal 

 production of acids in living protoplasm, and these acids pro- 

 duce, as a secondary effect, the physico-chemical change now 

 under consideration. These cytolytic substances are, as we 

 saw in Chap. IX., just those substances which break enudsions. 

 Egg protoplasm is an emulsion very rich in fat, and it is obvious 

 that the breaking of such an emulsion would lead to the setting 

 free of protein and would probably change the nature of the 

 complex from aqueous protein-in-oil type to an oil-in-aqueous- 

 protein type. The protein and lipoids carried to the surface 

 and coming in contact with sea-water would readily be adsorbed 

 and form a membrane (Chap. XI.). 



Eggs undergoing artificial parthenogenesis quickly show dis- 

 integrative changes unless means are taken to confine the cytolytic 

 effect to the surface. This is provided for in some cases [e.g. 

 starfish and certain annelids) by the specific nature of the proteins 

 in the cortical layer. The diffusion of the acid causes them to 

 alter in electrical state. They imbibe water, swell up and develop 

 normally. 



II. Exosmosis. 



In most cases, however, unless a second alteration is made in 

 the bathing medium, the egg will either not develop at all, or 

 will die at some intermediate stage. It is known that after 

 fertilisation the electrical conductivity of the egg is increased 

 for 15 minutes or so. This may be interpreted as a sign of 

 increased permeability of the membrane to surrounding salts, or 

 it may, with equal justice, be accounted for by a withdrawal of 

 water with a consequent increase in the concentration of electro- 

 lytes. During this period of increased electrical conductivity 

 the eggs readily undergo plasmolysis if placed in a solution of cane 

 sugar. Unfertilised eggs, of course, do not show alterations at 

 this early stage in electrical conductivity, nor are they so easily 

 plasmolysed. This may be taken as a confirmation of the second 

 hypothesis, viz. that water is removed by exosmosis and, con- 

 sequently, the concentration of electrolytes in the egg is increased. 

 In order to induce an exosmotic flow, the membrane must be 

 impermeable to sugar. If the membrane had increased in per- 

 meability during fertilisation the fertilised egg would probably be 

 less easily plasmolysed than the unfertilised egg (see Chaps. V. 

 and XI.) In addition, it is probable that an alteration takes place 

 in the ratio of the free to the bound water in the egg, and that 



