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92 ORGANIZATION OF THE INFUSORIA.that corresponds most closely with this importantfunction as it is metwith in all Metazoic or multicellular structures. As already maintainedin the description of that more common phase of multiplication whichtakes the form of constantly repeated binary division, the Infusoriathrough such mode of increase merely repeat in a separate and inde-which characterises thependent manner that process of cell-multiplicationnormal growths of all tissue structures. By-and-by, however, in the case ofthe Metazoon or tissue organism, an epochis arrived at when the componentcells cease to exhibit their previous duplicative energy, the consequencebeing the gradual decay and ultimate dissolution or death of theentire organism. But for the interposition of a special and more or lessperiodical regenerative act, this termination of the life of the individualwould also, sooner or later, involve the extinction of the race or species.Such a regenerative act, and the further survival of the race, is, however,here accomplished through the fusion or union of one of that congeries of cellsout of which the compound organism is composed with one other exteriorlyderived cell liberated from another organism or congeries of cells pertainingto the same specific type. The result of such fusion between these two,denominated respectively the germ-cell and sperm-cell, or, in other words,the ovum and spermatic cell or element, is the capacity conferred upon theformer of once more proceeding with the duplicative process, and repeatingthat cycle of cell-aggregation or tissue-construction followed by the parentorganism. In a precisely similar manner, the infusorial body, after repeatedduplicative multiplication, arrives at a condition in which the strain orrace is too exhausted for the further maintenance of this process, andwithout the intervention of some supplementary regenerative operationwould become extinct. By, however, the coalescence or fusion of one ofthese individual cellsor animalcules with an elemental cell or animalculederived from a neighbouring race or strain, the capacity to continue theduplicative process is revived, and the further duration of the race secured.The two animalcules thus uniting with one another correspond to all intentsand purposes with the coalescing germ-cell and sperm-cell, or ovum andspermatozoon of the higher tissue organisms, the only essential point ofdivergence being in the subsequent changes manifested ; the organism inthe latter instance exhibits a tendency to build out of the cells amassedby the duplicative process, more or less complex coherent tissues, while inthe former each cell so produced maintains a separate and independentexistence.Among the extensive series of types included in the infusorial classes, thephenomenon of " conjugation," " fusion," or " zygosis," as it is variously called,and which undoubtedly represents the sexual or genetic reproductive processof the higher animals in its most simplified or elementary form of expression,exhibits certain well-defined modifications. With the majority of the morehighly organized Ciliata, such conjugation has to be denominated, as comparedwith the form next described, transient and incomplete, being, as