CELL KEPRODUCTION 7 



possess distinctive features of their own. The cell may be justly regarded 

 as an individual, whether it is one of the Protista or only part of the body 

 of a multicellular organism. Tn the latter case it must be admitted 

 that a number of individuals have remained united as a colony to form 

 a single larger individual. Of the cells of the latter, only certain ones are 

 destined for reproduction, as in the case of spores of Cnidosporidia, 

 where a group of cells is formed by division from a single cell, and of 

 these only one is a reproductive cell, the others dying after fulfilling other 

 functions. A single soldier or a regiment of soldiers may both be units 

 in the military sense, but the soldiers composing the regiment, though 

 sacrificing their individuality to some extent for the good of the individual 

 regiment, are as much individuals as the single soldier. 



It has been clearly demonstrated that a Protozoon quickly dies if 

 deprived of its nucleus, and there is little doubt that the cells of higher 

 animals are similarly dependent on their nuclei. A single unicellular 

 organism may be divided into several portions, but though those which 

 do not contain the nucleus may exhibit movements and survive for some 

 time, they ultimately die, whereas any nucleated portion may re-form 

 itself into an entire individual which is able to continue its existence. 

 It is evident the nucleus plays a very important part in the life, and 

 metabolism of the cell. The Protozoan cell does not differ from other 

 cells in its capacity to absorb and digest food, and grow and increase in 

 size. It is able to perform spontaneous movements as a result of con- 

 tractions of its cytoplasm, though these are reduced to a minimum in 

 some cases. Finally, the cell is able to multiply, usually by a process 

 of binary fission, but sometimes by a process of multiple fission. In 

 binary fission the single nucleus divides into two parts, and this is followed 

 by division of the entire cell into two daughter cells. Usually, these 

 are approximately equal in size {equal binary fission), but it may happen 

 that one daughter individual is larger than the other {unequal binary 

 fission). When the difference in size is marked, it appears as if a small 

 daughter individual is separated from a much larger parent which retains 

 its original form, and the process is spoken of as budding or gemmation. 

 In the case of multi'ple fission or multiple segmentation, after the first 

 division of the nucleus the body of the organism does not immediately 

 divide, but the two daughter nuclei again divide to form four nuclei, and 

 these may again divide to give rise to eight. After a number of nuclei 

 have been thus produced by repeated divisions, the body of the organism 

 segments into, or more accurately buds off, a number of portions cor- 

 responding to the number of nuclei. This method of multiple fission of 

 cells, which more correctly should be called multiple gemmation, occurs 

 in higher animals as well as in the Protozoa, amongst which it is seen 



