26 



THE CELLi AND PROTOPLASM 



of Valonia ventricosa, a coenocytic, marine 

 alga. Aplanospores are obtained by punc- 

 turing a turgid coenocyte. The ruulti-nu- 

 cleate protoplast separates into numerous 

 mono- or poly-nucleated amoeboid bodies 

 which become spheroidal about 10 to 15 

 minutes later. The amoeboid fragments 

 show a high coalescency (Kopac 1937). 

 Within half an hour of their formation, the 

 aplanospores, now spheroids, develop a po- 

 tential barrier at their surface which is no 

 greater than that of the assumed naked 

 Echinoderm eggs. "With each succeeding 

 half-hour the barrier increases in its ef- 

 fectiveness, indicating the gradual develop- 

 ment of an extraneous coat. About four 

 hours later, these spheroidal aplanospores 

 are invested in a definite cell wall and no 

 coalescency occurs. 



All oil-protoplasm interfacial tensions so 

 far measured are of a low order of magni- 

 tude. For the naturally occurring oil 

 drops in the mackerel and Daphnia egg, 

 Harvey and his coworkers (Harvey and 

 Shapiro 1934; Harvey and Schoepfle 1939) 

 have found values of the order of .1 to 2.6 

 dynes per cm. Their method was to calcu- 

 late the interfacial tension from the degree 

 of flattening of an oil drop inside the eggs 

 under a given centrifugal acceleration. 

 Kopac (unpublished) obtained values of 

 10"^ dyne per cm for various oils against 

 Arhacia egg protoplasm by using a micro- 

 adaptation of the maximum bubble pressure 

 method. 



Living cells (aging Arhacia eggs, for ex- 

 ample) will coalesce with oil drops having 

 an interfacial tension against sea water of 

 2.5 dynes per cm. Therefore the oil-proto- 

 plasm interfacial tension must be lower 

 than 2.5 dynes per cm. Coalescence is 

 thermodynamically impossible if the oil- 

 protoplasm interfacial tension is higher 

 than the oil-aqueous phase interface. 



The oil capping reaction which was first 

 observed for fresh-water amoebas by Daw- 

 son and Belkin (1929), can also be made to 

 occur on protoplasmic exovates of starfish 

 eggs and naked sea-urchin eggs. The oil 

 drop comes to equilibrium by partially 

 flattening against the cell surface. Such 

 flattened drops can be released only after 



cytolysis is induced, whereupon the oil 

 drop assumes a spherical shape. "With the 

 Echinoderm egg, it has been found that the 

 oil cap can be made to slip over the surface 

 of the egg in a way similar to the slippage 

 of an oil cap on the surface of water. A 

 spontaneous capping involves a reduction 

 in potential energy of the applied oil drop, 

 since the flattened drop now possesses 

 greater surface area than the original sphe- 

 roidal drop (Chambers and Kopac 1937). 

 The following conditions must exist in the 

 capping reaction: (a) tangential rigidity at 

 the cell surface must be sufficient to prevent 

 coalescence, and (&) the tension at the oil- 

 cell surface interface must be lower than 

 at the oil-aqueous phase interface in order 

 to compensate for the increase in area of 

 the oil cap over that of the applied 

 spheroidal oil drop. 



The interfacial tensions between oil and 

 internal protoplasm,^ although low, are 

 nevertheless always positive. This conclu- 

 sion is to be inferred from the following 

 facts : 



(a) All natural oil drops, namely, those 

 occurring normally in cells, are spherical. 



( h ) Experimentally introduced oil drops 

 are also spherical whether the oil be polar 

 (plant and animal) or apolar (mineral). 

 Any deformation can be accounted for by 

 the presence of some mechanical obstruc- 

 tion. An interesting case is the ovoid 

 shape of an oil drop introduced into the 

 radially gelated aster of a fertilized sea- 

 urchin egg. The drop maintains its dis- 

 torted shape as long as the aster is present. 

 "When the gel reverts to the sol state either 

 by agitation of the aster with a needle or 

 in the natural succession of events in the 

 cell, the oil drop reverts to a spherical 

 shape. 



(c) Spherical oil drops deformed in the 

 cell by centrifuging return to the spherical 

 shape when the centrifugal force is re- 

 moved. 



(d) An oil drop constricted in the equa- 



3 The foregoing discussion is based on some pub- 

 lished data and also on various unpublished data 

 accumulated in this laboratory on oil-protoplasm 

 interfacial phenomena. Detailed accounts are in 

 preparation for early publication. 



