PROTOPLASMIC FUNCTION 39 



streams, or flows, into this pseudopod, as it is called, the ameba, when 

 this is done, hav ng travelled its own diameter. 



Often when one looks at the animal on the slide of a microscope there 

 is no such progression in process, and several pseudopod ia may be 

 extended in several directions at the same time, all different in shape and 

 size, save by accident, but all formed by this same peculiar streaming, 

 rolling form of motion characteristic of the liv ng substance. The strik- 

 ing impression which it gives is that the whole mass all through is equally 

 alive and equally active and helpful in the general heaving and flowing 

 movements of the pseudopodia s owly back and forth. Sometimes the 

 movement is aimless, sometimes after food, but it seems almost as if the 

 motion were inherent in the substance, pervading it thoroughly in every 

 part. That is the so-ca led ameboid movement, difficult to describe, but 

 fascinating and instructive when seen under the microscope. This is the 

 basal, actively vital movement of the almost undifferentiated protoplasm, 

 automatic and, in a sense, inseparable from the nature of the living sub- 

 stance save at special periods when certain animals may become encap- 

 sulated and rest. The physical principles on which these movements 

 depend are in doubt, but the principle of surface-tension in liquids is 

 probably the most important of them. Kiihne, in 1864, showed that the 



FIG. 13 



Diagram of a row of cilia to show the rhythmic nature of the contraction. (Verworn.) 



extension of a pseudopod represents a decrease of the surface-tension at 

 the point where it projects. He supposed this decrease to be caused by 

 the absorption at that spot of some of the surrounding oxygen, the 

 cohesion of the biomolecular atoms being lessened by the admission of 

 these atoms of oxygen into the living molecule. The retraction of the 

 pseudopod is accounted for by supposing that the absorption of the 

 oxygen at that point causes an increase of surface-tension which retracts 

 the pseudopod. Verworn's further supposition is that the mass of mole- 

 cules thus more or less exhausted of their energy is restored to the vigor 

 necessary for activity by metabolic interchange with the nucleus of the 

 cell. Quincke showed, however, that oil drops in an alkaline liquid have 

 an action similar, under certain conditions, to that of drops of living 

 protoplasm. Biitschli's researches already mentioned are conclusive 

 that some at least of the phenomena of protoplasm may be imitated with 

 inorganic materials. 



The second class of movements in protoplasm due to its active con- 

 traction and passive relaxation consists of those called ciliary. A cilium 

 is a thread-like projection from the protoplasm of certain cells, while a 

 flagellum is an enlarged and somewhat elaborated cilium. Cilia appear 

 to be present in nearly all animals above the very lowest, either on the 



