256 IRRITABILITY 



in the stretching out of the pseudopods by a centrifugal outflow- 

 ing of the protoplasm into the surrounding medium, the contrac- 

 tion phase by the indrawing of the pseudopods by the centripetal 

 inflowing of the protoplasm to the cell body. In total contraction, 

 such as occurs, for instance, in strong excitation following stimuli, 

 the cell body becomes ball shaped. In local contraction of the 

 long thread or net-shaped outstretched pseudopods of the sea 

 rhizopoda, the protoplasm of the retracting pseudopod forms balls 

 and spindles. Considered from a physical point of view the 

 expansion phase of amoeboid movement is an expression of de- 

 crease, the contraction phase an increase of the surface tension. 

 I have shown that the factor which under physiological conditions 

 decreases the surface pressure and thereby brings about the 

 expansion phase is the introduction of oxygen into the living 

 substance. With removal of oxygen the stretching out of the 

 pseudopods ceases. The cell gradually draws in all pseudopods 

 and assumes the shape of a ball. On the reintroduction of oxygen 

 the outflow of the pseudopods begins anew. This fact can be 

 observed in all amoeboid cells. When, therefore, consumption of 

 oxygen and oxydative changes is suppressed during narcosis it is 

 to be expected that all naked protoplasm masses by being nar- 

 cotized lose their capability of assuming the expansion phase of 

 movement and contract into the shape of balls. Experimentation 

 confirms this deduction in the most striking manner. When 

 amoebae are placed in a drop of water under the microscope in a 

 gas cell through which air and a little ether are allowed to flow, 

 the pseudopod formation of the amoebae ceases within a few 

 minutes and they all assume the shape of a ball. (Figure 62.) In 

 asphyxiation in pure nitrogen, the changes in the amoebae take 

 place in exactly the same manner with the exception that in this 

 case a longer period ensues according to the size and activity of 

 the animals. About 20 to 60 minutes elapse before depression 

 becomes complete. If larger sea rhizopoda are narcotized in the 

 same manner all pseudopods are more or less retracted and the 

 contained protoplasm flows centripetally and contracts in the 

 characteristic manner into balls and spindles. (Figure 63.) If 

 the narcosis is removed by displacing the ether by pure air, the 



