

24 THE CELL 



2. Tin Protoplasmic or Plasmatic Membranes. 



Every protoplast is surrounded by a special dermal layer, the 

 external plasmatic membrane or ectoplast. Where this membrane is 

 visible at all, it appears to consist of hyaline non-granular protoplasm, 

 the so-called hyaloplasm: the granular protoplasm or polioplasm 

 enclosed within the ectoplast, on the contrary, presents a more or less 

 turbid appearance owing to the presence of numerous granular or 

 vesicular inclusions, the microsomes. The protoplast is likewise 

 separated from the various vacuoles which it contains by hyaloplasmic 

 membranes, the vacuolar or internal plasmatic membranes. 1 ' 



The ectoplast consists of relatively solid protoplasm, and is possessed 

 of a special structure, Iry virtue of which it is able to control many of 

 the relations which the protoplast maintains with the outer world. In 

 the case of naked protoplasmic bodies, such as the plasmodia of 

 Myxomycetes or the swarm-spores of Algae, the ectoplast serves princi- 

 pally for protection, a function elsewhere performed by the cell-wall. 

 Further, whether the protoplast is clothed in a cell-membrane or not, 

 the ectoplast invariably regulates the osmotic interchange with the 

 external medium or with adjacent cells. Pfeffer's researches in pai 

 ticular have made it clear that this membrane controls both the 

 entrance of dissolved substances required for the nutrition, and in fact 

 for the general metabolism of the cell, and the egress of bodies manu- 

 factured by the protoplast. The osmotic interchange of material does 

 not proceed entirely in accordance with physical laws, but is also 

 influenced to a great extent by the physiological requirements of the 

 cell. The specific structure of the plasmatic membranes is adapted to 

 these requirements; it is for this reason that a substance of relatively 

 high molecular weight may be able to penetrate freely through the 

 ectoplast, while passage is denied to the far smaller molecules of 

 another compound. These statements are equally applicable to the 

 internal plasmatic or vacuolar membranes, the physiological behaviour 

 of which has been particularly studied by de Vries. 



The ectoplast is also closely concerned with the production of the 

 cell-wall : indeed, in view of the fact that it lies in close contact with 

 the cell-wall while the latter is undergoing growth and differentiation, 

 there can be no don lit that this plasmatic membrane is directly involved 

 in both these processes. Again, when a limited region of a cell-wall is 

 iiii leasing or diminishing in thickness, the underlying portion of the 

 ectoplast must take an active part in the change, since all such processes 

 of growth or disorganisation are under the immediate control of the 

 living substance. As Noll points out, it is advantageous in this 

 respect that the ectoplast is relatively solid and stationary, and 



