382 THEORETICAL RESPONSE OF CELLS TO CONTACT 



reached at complete ingestion until the surface tension is such that the 

 cell will spread to infinity. In the lower five curves in Fig. 4 the sur- 

 face energy is less at complete ingestion than before ingestion; i.e., 

 Tait's condition is fulfilled, but the particle is nevertheless only par- 

 tially ingested except in the lowest curve. 



From this point of view the surface of every particle, however 

 small, must be regarded as an infinite surface, just as a flat surface 

 may be regarded as a curved surface with a radius infinitely large. 

 The only predictions which we can make from principles of surface 

 tension are: 



1. If a cell spreads to infinity on a flat surface of G, it will completely 

 ingest a particle of G. 



2. If a cell does not stick to a flat surface of G, a small particle of G 

 will not stick to the cell. 



3. If a cell is partially spread out on a flat surface of G, a particle 

 of G will be partially ingested. 



Tait has predicted from his discussion that: "If a substance, C, is 

 unstable on a slab of G, then C will ingest G." This is true if "un- 

 stable" means spreading to infinity; but when he tries to prove this 

 prediction experimentally by observing that hyahne thigmocytes 

 are unstable on glass and ingest small particles of glass, it may fairly 

 be objected that he could not have observed the cells spreading to 

 infinity and that the prediction consequently is not fulfilled by the 

 facts. The explanation of the experiment is either that the cell was 

 too rigid to spread to infinity on a flat surface though it could do so 

 readily on the curved surface of a small particle, or that the equilib- 

 rium observed on a flat surface was a true surface tension equihb- 

 rium and that the small force of surface tension which prevented the 

 small particle from being completely ingested was easily overcome by 

 the ameboid movement (changes in consistency) of the cell. 



A similar explanation may be given when Tait predicts that: "If 

 C ingests a particle of G, then C (may or) may not be unstable on a 

 slab of G," and adds in consequence that "the fact that stable cells 

 such as mammalian polymorphonuclears do not spread extensively 

 on glass is no evidence that they ingest by other than physical 

 means." The true prediction from principles of surface tension is that 

 if C ingests G, it will spread to infinity on a slab of G, and the ob- 



