32 Cuti,i?r, Parthenogenesis in .Animals 



cyclic coagulations and liquefactions, which are supposed to occur 

 tin, a developing egg s have never been satisfactorily demonstrated, 

 and until this lias been done the theory must of necessity remain 

 a speculation. 



Theory of Lillie. 



This theory has analogies with the preceding one. Lillie 

 maintains that in the egg there is a latent mechanism, which 

 can be set into action by various substances. While Loeb con- 

 siders that this is caused by intra-cellular oxidations set in motion 

 by catalyserSj Lillie takes up the position that the action is due 

 to increased permeability of the egg membrane and not to any 

 specific chemical substance. Development is always preceded by 

 mitotic divisions of the cell, and as there is evidence that during 

 this di vision there is a change in the cell permeability, Lillie 

 concluded that this is the direct agent tin artificial partheno^ 

 genesis. In iqio he wrote: "The egg is to be regarded from 

 a simple physico-chemical point of view as a chemically complex 

 semi-fluid colloidal system, enclosed by a semi-permeable surface 

 layer, the plasma membrane, which is the seat of electrical polari- 

 sation. Increase in permeability will evidently produce both 

 chemical and physical changes in such a system,; the chemical 

 changes follow from altered conditions of interchange with the 

 surroundings, as already seen, and involve disturbances of 

 chemical equilibrium in the egg; these, latter, on the present 

 theory, initiate the chemical transformations which find expres- 

 sion in the mitotic process. The chief physical changes from 

 the present point of vie-wi Avould be ;a decrease in the electrical 

 potential difference normally existing between the exterior and 

 interior of the cell. The seat of (this potential difference on the 

 membrane theory is the plasma membrane which appears to be 

 electrically polarised in such a way as to have its outer surface 

 constantly at a considerably higher potential than its inner; tins 

 condition, the physiological polarisation, is a function of the im- 

 permeability of the plasma membrane to ions other than certain 

 cations, probably hydrogen ions. Here more or less complete fall 

 of potential. i.e t depolarisation, must follow an increase of sur- 

 face permeability sufficient to allow ready passage of anions, 

 such depolarisation will be accompanied by increased surface 

 tension. Alteration of surface tension thus induced form, in all 

 probability, an important, if not the chief factor, in the charac- 

 teristic changes of cell cleavage." 



This conclusion was deduced from experiments, of which the 

 following are examples : — 



Unfertilised eggs of Arbacia punctulata were found to lose 

 their pigment when placed in isotonic solutions of various salts. 



