PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 89 



small granules of deeply staining substance, the centrosomes. The 

 centrosomes, spindle fibers, and chromosomes, to which the spindle 

 fibers are attached, are collectively known as the mitotic figure, and 

 few cells that are known divide without the formation of this mitotic 

 figure, or some modification of it (Fig. 27). It represents, therefore, 

 the mechanism of cell division, and further, since the hereditary char- 

 acteristics are now known to be connected in some way with the 

 chromosomes, the mitotic figure becomes the mechanism of heredity. 

 The chromosomes, while in the equator of this mitotic figure, or in 

 some cases even before the mitotic figure is formed, are divided by a 

 cleft which passes from end to end through the centre, and the two 

 halves, as the daughter chromosomes, are apparently drawn apart by 

 the mechanism of the mitotic figure; the cell body is then divided 

 into two daughter cells by a constriction or cleft passing through the 

 middle; the nuclei reform their characteristic reticular condition, and 

 the two cells are then ready for further processes of digestion, assimi- 

 lation, and growth. 



Ever since 188.3, when Roux first called attention of biologists to 

 the extreme care with which the chromosomes were halved and dis- 

 tributed to the daughter cells, and especially since the publication of 

 Weismann's classical essays on the nature and constitution of the 

 germ plasm, these elements of the cell have been recognized as the 

 physical basis of inheritance, and their mode of origin and complete 

 history have been the chief subject for study by cytologists. Not only 

 the chromosomes, but the entire spindle figure as the mechanism by 

 which they are divided, has also demanded the attention of biologists. 



In this branch of biological research the protozoa have played an 

 important part, for in these cells we find the simplest types of the 

 division figure and the simplest forms of the chromosomes, while cell 

 division is found in every conceivable form, sometimes strikingly 

 similar to the division of a metazoan cell, as in some heliozoa, some- 

 times so highly modified as to be regarded as a type by itself, as in the 

 budding forms. 



Cell division, therefore, which Spencer interpreted as marking the 

 limit of growth of a cell, is inaugurated through some change in the 

 relations of nucleus and cytoplasm, and some change which is entirely 

 unknown. In many protozoa the process is so different from tissue- 

 cell division that other names are given to it. We recognize : (1) Simple 

 binary division of the cell into equal parts, or simply cell division. 

 (2) Unequal division of the cell, the smaller part being pinched oft' 

 from the larger as a bud. This is known as budding or gemmation, 

 and is only a slight modification of cell division. (3) Spontaneous 

 division of the cell into four or more, frequently a great number of 

 daughter elements, each with a portion of the original cell nucleus, the 

 process being known as spore formation or sporulation. 



