The method of binary fission or splitting, by which the body of the parent becomes divided into two equal parts, into halves, is the simplest method of multiplication. This method is made use of in the amoeba. In this kind of reproduction there is no parent nor child. The children, the new amoebae, are simply the parent cut in two (vide Page 60). The next simple generation is budding, which is the breaking off of a part smaller than half from a certain individual. The budded-off part has the capacity of growing into a new individual like the parent. This mode of reproduction is found in the hydra. Another simple mode of generation is that of sporulation. Here the interior of the body of the individual subdivides into more than two parts. Sometimes the parts number many hundreds and are called spores. These three methods are the simplest modes of generation and are exclusively found in the lowest forms of unicellular organisms. When we go a step farther, in the class of unicellular protozoa, the simple mode of multiplication continues, in most forms, only for a certain number of generations. Then the necessity for conjugation, i. e., for a temporary or permanent fusion with another individual, sets in. If this conjugation be prevented, the animal soon degenerates and dies.[W] [Illustration: CUT XXXV. Infusorium reproducing by sporulation.] The simplest terms of conjugation are found in Chilodon, a minute fresh-water infusorium, which multiplies for a considerable length of time by transverse division. After a time, however, the physiological necessity for conjugation sets in. The different animals place themselves side by side, in pairs, and partly fuse together. The nucleus of each individual divides now into two portions, one of which passes from each infusorium into the other to unite with the half of the nucleus that remained stationary. The two animals then separate, each having received a half of the nucleus of the other. Thereupon a period of renewed activity for each ensues, manifested by rapid growth and multiplication by division, until a certain weakening in the vital activities indicates the periodically recurring necessity for conjugation. The next step in conjugation is found in those animals in which after their fusion the two animals do not separate any more, as found in the fission of monads, which is preceded by the absorption of one form by another. One monad is fixed upon the sarcode of another, and the substance of the lesser, which is the lower one, passes into the upper one. In about two hours the merest trace of the lower one is only left, and in four hours fission and multiplication of the larger one has taken place.