JO 



PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 



alone, we find a far more satisfactory state of affairs, for the details of 

 the process are known in many different cases. 



Whatever the causes of cell division may be, whether limits of 

 growth or sun spots, the fact is established that the first indications of 

 the process in the majority of cases are found in the nucleus. Here we 

 are dealing with a universal biological phenomenon, the division of 

 a cell, and the protozoa are interesting in this connection because of 

 the variations in the process which they present, and also because the 

 structures involved are less complicated than those of higher animal 

 and plant cells, and, therefore, more easily analyzed. In all tissue cells 

 of normal character, division is brought about through the medium of 

 a peculiar structure of the nucleus known as the mitotic or karyo- 

 kinetic figure. Under ordinary vegetative conditions of the cell, the 



Fig. 27 



A micronucleus of Paramecium aurelia in division. 



nucleus contains chromatin substance in the form of granules arranged 

 in a more or less definite network or reticulum. Prior to cell division 

 these granules become rearranged in a much wound thread or spireme, 

 and later the spireme thread is divided across ii»to a number of short 

 chromatin elements known as the chromosomes, the number of such 

 chromosomes being constant for all of the cells of the same species of 

 animal or plant. The numl)er of these chromosomes in no way 

 indicates the degree of differentiation of the organism, nor its position 

 in the animal or plant scale, some protozoa, for example, having a 

 larger number of chromosomes than does man. In the ordinary 

 process of mitosis these chromosomes are arranged in the centre of a 

 spindle-formed nuclear figure consisting of fibers of kinetic substance 

 focussed at two poles, these poles characterized by the presence of 



