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GFNERAL BIOLOGY 



For every kind of cell there is a normal size, which being 

 attained, nucleus and cytoplasm act conjointly to bring 

 about a separationof the cell body into two equal parts, and 

 to perpetuate in each of the descendant cells the substance 

 and the characters inherited from past cell generations. 



When cells after division remain in contact they tend to 

 form individuals of a higher order. These may be merely 

 colonies of loosely associated and physiologically independ- 

 ent cells — mere aggregates — or, if the cells become inti- 

 mately associated in relations of mutual dependence, then 

 each aggregate becomes a unit organism. In organisms of 

 this compound sort new individuals may be formed by 

 external agencies. The filament of Spirogyra, for instance, 

 may be broken into a number of parts, and each part, pro- 

 vided it contain an uninjured cell, may become a new fila- 

 ment. New individuals of such 

 branching types as Cladophora or 

 Dinobryon are formed when the 

 older parts connecting branches to- 

 gether meet v/ith accident or death 

 and the connection is dissolved. 

 These are negative processes, that 

 do not account for the producti n of 

 anything new; it should be clearly 

 recognized that cell division is the 

 universal mode of increase among 



organisms. 



Fig. 66. Cercomonas (in part 

 after Dallinger). a, divi- 

 sion; b, a normal individual; 

 c, an individual approach- 

 ing the time for conjuga- 

 tion; d the begnning of con- 

 jugation; e, end of conjuga- 

 tion and fusion of nuclei. 



2. Sexual reproduction. — In a 



few minor groups of the smaller 

 organisms, cell division goes on un- 

 interruptedly, and is the only 

 known phenomenon of reproduc- 

 tion; but in all the larger and 

 higher forms of life (and in so 

 many of the simpler ones also that 



