100 The Structure of Protoplasm 



As regards plant cells it was through the investigations of my 

 co-workers, Siminovitch and Levitt, on cytoplasmic behavior in 

 relation to frost resistance that the widespread occurrence in such 

 cells of the same differentiation into gelatinous cortex and liquid 

 endoplasm or mesoplasm was impressed upon me. In a non- 

 vacuolate type, the pollen mother cells of Trillium, an outer plas- 

 magel zone of variable thickness and an inner fluid one are easily 

 demonstrated with the aid of a little micromanipulation (Stern). 

 In vacuolate cells having a thick layer of cytoplasm, such as the 

 cortical cells of many trees especially in winter time, the same dis- 

 tinction is apparent when rupture occurs during deplasmolysis. A 

 thin cortical layer tears the rest spills out and mixes with water. 

 Even in the thinnest cytoplasmic layers, this differentiation is 

 revealed by a technique based on Chambers' method of applying 

 oil drops to the cell surface. An oil drop snaps into the surface of 

 a plant protoplast, taking the form of a biconvex lens. When the 

 protoplast is stretched (by deplasmolysis or pressure) the oil drop 

 is pulled out into a flatter and ultimately concavo-convex shape. 

 When again the protoplast is allowed to contract, the oil goes 

 through reverse change in shape, and if the contraction goes far 

 enough it becomes spherical. Evidently the tension on the edge 

 of the oil lens is that of an elastic, not a liquid, film. But while 

 the cortex is thus shown to shrink and stretch elastically, the meso- 

 plasm flows into and out of the angular space between the edge 

 of the drop and the surface of the vacuole, and must therefore be 

 quite fluid (Siminovitch and Levitt) . 



PROTOPLASMIC SURFACE FILM OR ECTOPLAST 



The cortical gel layer, when wide enough to be seen, often 

 shows little optical differentiation from the liquid endoplasm ex- 

 cept as regards restriction of Brownian movement. Both zones 

 are commonly granular. In contrast to this there is distinguish- 

 able on the surface of many cells a thin layer of more hyaline 

 appearance. This has been called ectoplasm or the ectoplast, but as 

 the term has also been applied to the cortical gel in amoeba. Cham- 

 bers prefers to call it the "protoplasmic surface film." Much of our 

 knowledge of this layer comes from Chambers' studies of echinoderm 

 eggs. He gives many proofs of its fluidity and shows that its con- 

 tinuity over the surface is essential to the integrity of the proto- 

 plast, whereas other membranes which normally exist external to 



