THE INTERPHASE CELL AND THE CYTOPLASM 



of small granules may then be seen by direct observation. Mito- 

 ciiondria exhibit a very complex example of protoplasmic streaming, 

 the customary theories about which have been suggested as explana- 

 tions for this motion; but no real attempt to analyse it has yet been 

 made. Some of the possible ways in which mitochondria can change 

 their form is illustrated by a sequence of photographs of a cell of 

 Xenopus in culture in Plate I (2). In this example, within a few minutes 

 one filament rolled up and rotated ; it then divided into two blobs, one 

 of which elongated and later became Y-shaped. The extreme mobility 

 of the mitochondria in these Amphibian cells suggests that they are of a 

 liquid nature. Schneider and Hogeboom^* ^^ believe that at the surface 

 of rat liver mitochondria is a semi-permeable membrane, one reason for 

 this view being the osmotic behaviour of the isolated elements. The mito- 

 chondria of mammalian cells in culture are much less mobile than are 

 those of Xenopus. If the latter are enveloped by a membrane, it must 

 have great plasticity,. 



The movement of mitochondria suggests that they are not moved 

 passively by the surrounding cytoplasm ; sometimes two nearby filaments 

 will move quite quickly relative to each other. A single granule may 

 suddenly migrate alone for a distance of ten microns or more. 



Rashevsky and his school have developed the theory that proto- 

 plasmic streaming in general is caused by diffusion forces which result 

 from continually varying concentrations of metabolites in the neigh- 

 bourhood of centres of chemical change within the cell (Rashevsky^") ; 

 there is now good evidence that in some cells enzymes are associated 

 with mitochondria and so continuous molecular changes at their sur- 

 faces may be inferred. It would be very desirable to test this hypothesis 

 by applying the equations of Rashevsky to data derived from observa- 

 tion of mitochondrial movement. Another and not necessarily incom- 

 patible suggestion is that electrical potentials are involved. 



It has long been considered that mitochondria play some special 

 role in cell metabolism; the general evidence from work in this field 

 is summarized by Bourne. ^^ They can be stained in the living cell by 

 Janus green; according to Lazarow and Cooperstein,^^ this is due 

 both to the reduction of the dye to a colourless leucobase by the 

 surrounding cytoplasm, and also to the action of the cytochrome system 

 of enzymes within the mitochrondria which keep it there in the oxidized 

 form. In tissue cultures, the vital staining of mitochondria by this dye 

 (Lewis and Lewis^^) and by methylene blue (Ludford^) has been 

 shown by these authors to be inhibited by cyanide. Lewis and Lewis 

 were able to demonstrate that the effect was reversible. 



Alethods of isolation — Within recent years, methods have been 

 developed for isolating mitochondria and other cell fractions by 



23 



