PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 87 



ties. This is shown by the behavior and history of enucleated frag- 

 ments, which, as we have seen, cannot digest food; other functions are 

 similarly crippled by removal of the nucleus, and movement itself is 

 greatly impaired. A contractile vacuole will reform and will contract 

 to a certain extent in enucleated protozoa, but it will not act normally 

 and soon ceases to contract, swelling then, with the continued addition 

 of fluids, until the cell bursts, as in the characteristic phenomenon of 

 diffluence. 



When the constructive activities of the protozoan body exceed the 

 destructive, and when the addition of new raw material exceeds the 

 waste, new protoplasm is added to the old and growth results. The 

 dimensions of the cell are increased in all directions, the increase taking 

 place in the fluid protoplasm apparently throughout all parts of the 

 cell at the same time, a process of growth by intussusception. The 

 mere accumulation of reserve food granules plays no part in growth, 

 all growth ceasing when the cell becomes packed with them, but must 

 take place only after the necessary constructive changes have con- 

 verted such reserve stores into protoplasm. Growth continues until 

 the cell has attained to a more or less definite, optimum size, and then 

 it divides into two or more small cells according to the species. 



The explanation of growth is one of the unsolved problems of 

 biology, and we get but little nearer the solution in the case of pro- 

 tozoan organisms than in the higher forms of life. We know, indeed, 

 that growth ceases with the elimination of the nucleus, hence, we 

 conclude that the nucleus is a necessary factor in the process. Growth 

 in the protozoa can be controlled in a variety of ways, and we know 

 that certain conditions of temperature, of density, and the like, are 

 necessary. While the explanation of the finer processes of growth is 

 far away, the solution of the problem of cell division is almost equally 

 remote, and no theory yet propounded satisfies the conditions as we 

 see them in the various forms of life. Spencer's theory of volume and 

 surface is very seductive; indeed, it may be a step toward the final 

 solution. Briefly stated it predicates that a normal relation exists 

 between the protoplasm and the nucleus of the cell, and, if the form 

 remains the same, this relation is disturbed by growth, for the surface 

 of the organism increases as the square of the diameter, while the 

 volume increases as the cube. Hence it results that the mass increases 

 faster than the surface which provides the means of interchange with 

 the environment (absorption and the like). The changed ratio of 

 surface to mass of protoplasm, according to Spencer and his followers, 

 brings about internal changes which result in cell division. But 

 after this theory is stated, we know nothing more about the ultimate 

 causes of cell division than we did before. When the nature of the 

 changes is understood, the reason for cell division will naturally follow. 

 Leaving aside the causes of cell division, and looking at the phenomena 



