44 PROTOPLASM OF PROTOZOA 



matic replacement of worn-out parts or the growth of new ones. Chemical 

 processes must often be definitely localized in a small region within the 

 framework of a single cell. However, visible differentiation usually does 

 not take place within a single cell in the Metazoa to such an extent as 

 in the Protozoa, where we have a whole series of special differentiated 

 organelles for performing particular functions. 



A protozoan is usually regarded as both an organism and a cell; in 

 the Metazoa protoplasmic differentiation tends to be irreversible, and 

 in Protozoa to be reversible, so that the most highly differentiated cells 

 ever observed are found as individual Protozoa. This extreme differentia- 

 tion is, of course, surprising to one accustomed to the usual structural 

 simplicity of metazoan cells, and is probably the main reason for the 

 position taken by Dobell (1911) and others who deny the cellular 

 nature of the Protozoa. Several degrees of permanency of differentiation 

 may be distinguished among the Protozoa, although of course the groups 

 are not mutually exclusive. Thus we may distinguish: (1) temporary, 

 completely reversible structures or differentiations of the protoplasm, 

 such as pseudopodia and spindle elements; (2) differentiations usually 

 irreversible and lasting throughout the life of the organism, which may 

 be the seat of active chemical energy changes, such as cilia, flagella, and 

 myonemes, or, with little chemical changes, such as morphonemes and 

 pellicle; and (3) differentiations formed by protoplasm which are 

 chemically distinct from it, such as shells and secretions. 



Every time a protozoan divides, conjugates, or encysts there is a 

 tendency toward dedifferentiation, followed by reorganization, so that 

 the individual emerging from these processes often has developed a 

 new set of structures derived from the fundamental structure. These 

 changes must be accompanied or caused by physical realignments of 

 protoplasmic elements, of which little is known. Perhaps in no other 

 phylum of animals can such a wide range of type of mitosis and 

 cytokinesis be found as exists in the Protozoa. The morphological de- 

 tails of the division process in these forms are fairly well known, and 

 evidence is continually accumulating which shows that mitotic and 

 cytokinetic mechanism does not differ fundamentally from that of 

 higher forms. 



In the Protozoa, as in other types of cells, little is actually known 

 regarding the initial stimulus that starts the organism to divide, to 



