50 PROTOPLASM AND THE CELL 



site side slowly flows out. These coalesce, and thus the piece of nutri- 

 tious matter is surrounded by a living net. Sometimes living animals 

 are thus entrapped. The cytoplasm then flows in upon it closely, and 

 the particle is soon in the interior of the ameba. Soon a vacuole is 

 produced about the particle, probably by a process of secretion from 

 the immediately surrounding protoplasm. The product of this secre- 

 tion, which fills the vacuole it has made, is undoubtedly a proteolytic 

 enzyme in a solution such as that which digests proteids in the more 

 highly evolved animals. While digestion is going on the nutritious part 

 of the food is absorbed and assimilated to the ameba's protoplasm. The 

 waste is excreted by a method homologous to that by which the food was 

 ingested the waste particles are flowed away from after they have been 

 gradually worked to the periphery of the animal. 



The effects of various physical agents on an ameba are instructive. 

 We may examine first as to heat. Reduction of the temperature of ameba 

 causes a decided slowing of the ameboid movements, and they cease 

 some few degrees above C. When raised above 20 C. or so, an 

 average room-temperature, the quickness and size of the movements 

 increase, reaching a maximum at about 30 C. At about 35 C. the pro- 

 toplasm goes into what is called heat-rigor, the coagulation being com- 

 pleted when the temperature is at 40 C. or a trifle higher. To light, the 

 ameba reacts under ordinary aquarium conditions, but seeks to avoid 

 it when too strong. It discriminates between degrees of light and shade, 

 and this is only a general protoplasmic function. If a weak current of 

 electricity be passed through the water surrounding the animal, its pro- 

 toplasm elongates (by electro taxis) and turns its long axis in the direction 

 of the current. Pointed pseudopodia of small size extend outward, 

 usually toward the anode. If the voltage of the electricity be excessive, 

 the protoplasm of the animal is broken up, disintegrated, and scattered 

 in a characteristic manner. 



Like all protoplasm, that of ameba requires oxygen for its metabolism. 

 It is regularly attracted by an excess of the gas, while if its oxygen supply 

 be cut off, the pseudopodial movements gradually cease and the animal 

 becomes more or less spherical, and enters into the condition known as 

 necrobiosis, or death-in-life, in which the vital functions surely but gradu- 

 ally cease. Occasionally an ameba may be seen to reproduce itself a 

 process lasting two or three hours by its usual method of amitosis or 

 simple nuclear division. (See page 44.) The nucleus may be seen to be 

 constricting in one diameter. This is followed by a similar process in 

 the cytoplasm in the same plane. Finally, after only a thread of cyto- 

 plasm connects them, the daughter-cells wholly separate. These new 

 cells, which are like their parent save in size, proceed in their functions 

 seemingly as the mother did before them links of a chain which 

 theoretically has no end the protoplasm composing them being in this 

 limited sense immortal. The ameba also has to conjugate at times with 

 another individual in order to maintain continuously the descent. 



We are now able to summarize some of the underlying facts and ideas 



