THE MITOTIC CYCLE 



the fixation mixtures used in cytological investigations on the Golgi 

 body would give the same result when discs of liver pulp were immersed 

 in them. Again myelin figures appeared within the cells, but their 

 formation was found to depend on a sequence of changes which follow 

 as the various ingredients of the mixture diffuse into the tissue at 

 differing rates. First the pH is gradually lowered, and then the electro- 

 lyte concentration slowly rises. Next, the myelin figures are formed and 

 are finally stabilized by slowly diffusing molecules such as OSO4. 

 These authors suggest that the necessity for comparatively slow fixation 

 in order to produce a typical Golgi network explains why tissue cultures 

 have not proved to be easy material in which to demonstrate such 

 structures. In living cells in tissue culture, and elsewhere (Baker^^), 

 no lipoidal network can be seen by phase-contrast. Yet owing to its 

 high refractive index, fatty material is revealed by this method more 

 readily than is any other cell component. 



Vacuoles within cells 



The number of cytoplasmic vacuoles within cells in tissue is very 

 variable. Sometimes cells with none are seen close to others which 

 show numerous vacuoles among the lipoidal inclusions. Where the 

 cell boundary consists partly of an undulating membrane, vacuoles 

 can there be seen to enter the cell by the process which Lewis^^ has 

 called 'pinocytosis' (Plate II (3)). 



Mitochondria 



The mitochondria of rabbit and mouse spleen cells in culture are 

 thin regular filaments, seldom more than lOpi long. Chick mitochon- 

 dria are less uniform, and sometimes bend into loops; their estimated 

 thickness from both the electron micrographs of Porter et alii^ and 

 from phase pictures is approximately o-25-o-40(ji. The mitochondria 

 of the toad Xenopus are larger and much more variable in form (Hughes 

 and Preston^ ^). When a chick or mammalian culture is observed with 

 the phase or the dark-field microscope at 37°C, one occasionally sees 

 a granule or mitochondrial filament move with an immediately appre- 

 ciable velocity (Plate I (i)), but continuous observation over some 

 minutes is necessary to follow the movement of most of them. In a 

 'speeded-up' film in which the time scale is shortened by a factor of 

 about ten times, the movement of all the cell inclusions becomes 

 immediately obvious. 



The two facts that are known about this kind of intracellular activity 

 are that it is something more than Brownian movement and is a function 

 of living protoplasm. If the cell is poisoned by any of a number of agents, 

 the normal slow movements can be inhibited, while Brownian motion 



22 



