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THE CONTINUITY OF THE RACE 



division. Fission is a typical method of reproduction in most protozoa, in 

 the bacteria, and in many unicellular as well as filamentous algae. 



Essentially similar to fission is the multiple fission, or sporulation, 

 exhibited by various Protozoa and Protophyta. Here the parent cell, 

 instead of dividing into two (binary fission), divides to form many minute 

 masses of cytoplasm and nucleus, each capable of growing into a full- 

 sized organism. Simple fission, whether binary or multiple, constitutes 

 asexual reproduction in unicellular organisms. 



Fig. 15.2. Conjugation and fission in the slipper animalcule, Paramecium caudatum. Not all 

 stages are shown. A, two individuals unite by their oral grooves (the groove leading to the 

 "cell mouth"). B, the large nuclei (macronuclei) begin to disappear, and the small nuclei 

 (micronuclei) divide. C, each micronucleus divides again, producing four daughter nuclei, 

 three of which degenerate. D, the remaining micronucleus divides again, and one member of 

 each pair thus formed migrates into the other Paramecium. E, the two paramecia separate 

 (only one is followed from here on), and the two micronuclei in each individual fuse (sug- 

 gesting the fusion of sperm and egg nuclei in metazoa). F, the fusion nucleus undergoes three 

 successive divisions, producing eight nuclei (suggesting the cleavage divisions in metazoa) . 

 G, four of these nuclei grow into macronuclei. H, the cytoplasm constricts in the middle and 

 again in each half. /, four small individual paramecia are produced, each with one large and 

 one small nucleus. (Modified from Borradaile and others.) 



Sexual reproduction. Another common type of reproduction among 

 protozoa and unicellular plants is conjugation and fission, i.e., fission that 

 is preceded by a temporary union or complete fusion of two individual 

 cells. In conjugation the two cells either (1) effect a mutual interchange 

 of nuclear material and then separate, or (2) fuse into a single cell, in 

 which event they may be regarded as two gametes fusing to form a zygote 

 in a process that closely resembles the fusion of egg and sperm in many- 

 celled organisms. In either case, the resulting cell or cells contain at least 

 nuclear material from both conjugants. Following conjugation, either 

 binary or multiple fission of the conjugated cells (or cell) takes place. This 

 differs from simple fission essentially in that the daughter cells contain 

 material (at least nuclear material) from two different parent individuals. 



