ORIGIN OF THE INDIVIDUAL 213 



the two new cells. Simple as this seems, the fission of Para- 

 mecium, for instance, involves considerably more than the 

 halving of the original cell, because, as a matter of fact, each 

 half must reorganize into a complete new individual with all 

 parts characteristic of the parent. (Fig. 11.) 



Among some unicellular organisms (e.g., Sphaerella) the 

 parent cell, instead of merely forming two cells by binary 

 fission, becomes resolved into from four to several hundred 

 cells by a series of practically simultaneous divisions known 

 as MULTIPLE FISSION, or SPORULATION. This is usually pre- 

 ceded by a considerable growth of the parent cell and its 



C D 



FIG. 114. Yeast cells, very highly magnified. A, cell showing granular 

 cytoplasm and a large vacuole; B, showing nucleus; C, cell budding; D, mother 

 cell and bud after division is completed. 



enclosure in a protective covering, or CYST, which ruptures 

 to liberate the spores. Other unicellular forms, such as the 

 Yeasts colorless plants chiefly responsible for alcoholic 

 fermentation exhibit a modified form of fission, in which the 

 parent cell forms one or several outgrowths, or BUDS, which, 

 gradually assuming the characteristic adult structures, are 

 usually detached as complete similar individuals. (Fig. 114.) 

 In a considerable number of instances, however, the cells 

 arising by multiple fission or budding remain closely asso- 

 ciated or organically connected so that they form a COLONY. 

 In some colonial organisms the component cells are all alike 

 and each retains its individuality, while in others certain cells 

 are restricted more or less in their functions, so that a phys- 

 iological division of labor is established which involves the 



