SHAPE OF ERYTHROCYTES'^ 315 



to conceive how differences of snrface tension could 1)e produced at 

 different parts of a cell surface hounding two fluids such as cell 

 contents and hlood plasma. 



3. Ponder's view is that the erythrocyte contains Huid or semi- 

 fluid material, and if the volume of the cell be increased by the 

 passage of fluid into it, the diameter (equatorial axis) must 

 diminish at the same time as the polar axis increases, until the 

 ratio of the axis is about 1 to 1-6. Thereafter both axes increase. 

 This investigator has worked out his theory mathematically from 

 the principles governing the stretching of clastic membranes of 

 spheroidal form and has constructed models which when distended 

 assume a discoid form. 



Experiments by Gough and by Seeker may be considered as 

 supporting this theory. The former's experiments also give a 

 clear indication of the intimate relationship between the red cell 

 and the blood plasma. On removing the last traces of plasma and 

 suspending the red cells in isotonic saline, they assume the spherical 

 form. The addition of serum, however, causes them to reassume 

 their discoid form. The volume of the cell in both forms is the 

 same. Seeker showed that unleashed corpuscles retained the 

 discoid form in various saline solutions, but if to any of the solu- 

 tions a small quantity of insulin or of guanidine carbonate solution 

 were added, the corpuscle became spherical. If the saline were 

 isotonic or hypertonic the sphere had a smaller diameter than the 

 disc. On the other hand, when a hypotonic saline was used, 

 although the sphere was of the same diameter as before, the 

 haemoglobin of these corpuscles gradually diffused out without 

 apparent rupture of the plasma membrane, leaving spherical 

 " ghosts." 



Now we must admit that, at all times, as long as the cell 

 membrane is intact, the contents of the cell must be in hydrostatic 

 equilibrium with the plasma. That is, the forces applied to the 

 cell membrane from without the cell must be exactly balanced by 

 the forces applied to the membrane from within the cell. The 

 forces tending to compress the cell are (i.) the elastic pressure of the 

 membrane; (ii.) the osmotic pressure of the plasma colloids 

 and of those crystalloids to which the membrane is impermeable. 

 On the other hand, the opposing forces are the osmotic pressures 

 of the cell colloids, viz. haemoglobin and cell-globulin ^8, and 

 possibly of some crystalloids like potassium. If this balance is 

 altered the cell will alter in size but not necessarily in shape. 

 An increased power of imbibition conferred on the cell colloids by 

 increased COg tension, for instance, leads to a swelling of the 

 corpuscle but no alteration in shape. Disturbance, however, of 



