862 THE MECHANISM OF PHAGOCYTOSIS 



and the ameba might have made itself evident as a slight repulsion, but this could 

 not have amounted to much in comparison with the energy being expended by the 

 ameba to cut the Paramecium in two. There is the possibility that a leukocyte be- 

 haves also in this way — that it receives something in the nature of a stimulus from 

 the particle to be ingested and makes a definite energy expenditure in response. It is 

 conceivable, then, that because of some chemical peculiarity of the carbon surface it 

 acted as a more effective stimulus than the quartz surface. But leukocytes can be 

 shown to carry on very active ameboid movements around quartz particles, some- 

 times seeming to have partly ingested them, but in the end frequently moving away 

 without them. The activity of the cell is, apparently, no greater in the presence of 

 carbon than in the presence of quartz. Its oxygen consumption would doubtless be 

 no greater. If this is so, the conclusion seems inevitable that carbon particles are 

 better ingested merely because, once they have come into contact with protoplasm, 

 there is less tendency for them to come away again. 



Still the observations do not quite prove that surface energy relationships are of 

 importance in determining the greater ease of ingestion of carbon, for it is conceivable 

 that the cell would find the smooth surface of a quartz particle too "slippery," for 

 example, while the microscopical roughness of the carbon afforded a more enduring 

 contact. This possibility can be disproved by a simple and striking experiment. If 

 the same field of leukocytes and particles described above could be examined in an 

 acid medium at pH 6,' it would be found that, for the same number of contacts with 

 the leukocytes, quartz was ingested perhaps twice as rapidly as the carbon particles. 

 It cannot be supposed that acid makes the quartz surface rough while the carbon 

 surface becomes smooth. The difference between carbon and quartz particles is evi- 

 dently something which can be reversed by acid, and this points strongly to surface 

 forces. Confirmatory evidence is derived from the behavior of carbon particles pre- 

 pared with the aid of gum acacia as a stabilizing agent. At all acidities such particles 

 are taken up more readily than quartz particles, and similarly it is found that the 

 cataphoretic potential can no longer be reversed by acid after treatment with acacia. 



With these carbon and quartz particles, however, we are dealing with a peculiarly 

 simple case. It seems to be merely a question of the partition coefficient of carbon and 

 quartz between the two phases— plasma and cells. Similar differences are made use 

 of in methods for separation of ores by flotation, and they can readily be demonstrated 

 by shaking up carbon and quartz particles with phenol and water, in which case 

 8 1 per cent of the carbon is found in the phenol and 88 per cent of the quartz is found 

 in the water. 



A case which is not so simple is that of the black particles of MnOa particles and 

 the transparent Mn silicate particles. Here the former are ingested at least 20 times 

 as rapidly as the latter; but careful observation and counts show that the leukocytes 

 are definitely attracted toward the former, for there are 2.4 times as many contact? 

 per unit time with the MnOj as with the Mn silicate, although present in equal num- 

 bers. Here there is evidently something like a stimulus which comes from the MnO, 

 particles and causes an extra liberation of energy from the leukocyte resulting in an 

 apparently "purposeful" progression in a definite direction and prompt ingestion. 

 I have had the good fortune to see some very beautiful and convincing moving pic- 



' Fenn, W. O. : loc. cil. 



