12 ZOOLOGY FOR MEDICAL STUDENTS chap. 



in size until at last it bears just about the same proportion to the cyto- 

 plasm containing it as it did to the original full amount of cytoplasm 

 before any had been cut away. And this appears to illustrate a general 

 principle namely that in any particular type of cell there is a fairly 

 definite normal proportion in size between nucleus and cytoplasm, and it is 

 suspected that the disturbance of this proportion may play an important 

 part in the production of certain abnormal conditions met with in disease. 



We have seen that the movements of the Amoeba are of a characteristic 

 type. Now it is an interesting fact that these movements can be produced 

 artificially in non-living substance. It is possible by using special 

 methods to produce extremely fine froth or foam composed of a mixture 

 of slightly rancid oil and watery fluid. Minute droplets of this foam 

 placed in water are seen when watched under the microscope to change 

 their form, " pseudopodia " being pushed out from their surface and the 

 whole droplet changing its position just as a live Amoeba would do. 

 But in the case of these oil droplets the movement is capable of physical 

 explanation. Any drop of fluid immersed in another kind of fluid with 

 which it does not mix is subject to the laws of " surface tension." Its 

 surface layer acts as if it were an elastic membrane always tending 

 to shrink in area : the tendency of the drop therefore is to assume a 

 spherical form — the form in which the proportion of surface to bulk is 

 at its minimum. If a portion of the surface begins to bulge out and form 

 a projecting " pseudopodium " the meaning of the phenomenon is that 

 that particular part has had its surface tension reduced so that it gives 

 way to the internal pressure due to the tension of the surface layer as 

 a whole. 



Now there is no reason to doubt that the pushing out of the pseudo- 

 podium of a living Amoeba is due similarly to weakening of the surface 

 tension over that particular portion of the Amoeba's surface. But it 

 must be borne in mind that this explanation of pseudopodium-formation 

 while quite satisfactory so far as it goes is not by any means a complete 

 explanation, for when we ask the question why should the surface 

 tension in some particular region undergo the diminution which leads 

 to the pushing out of a pseudopodium the only answer is that it is due 

 to some process involved in the living activity of the cytoplasm regarding 

 the nature of which we are quite ignorant. 



It is not merely the extrusion of pseudopodia which can be produced 

 in non-living substance : the same applies to the gradual ingestion of a 

 slender filament. If a drop of chloroform is brought into contact witli 

 a fine filament of glass-wool coated with shellac it is found that the surface 

 tension of the chloroform gradually draws the glass filament into the 



