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]
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.
From these two modes of generation it is easily seen that the impelling force leading to conjugation is, as Rolph puts it, simply cell hunger. These modes of multiplication are also found in the simpler forms of colonial protozoa,[X] where the cells are not yet differentiated and all the members of the colony take part in reproduction.
In the next higher class of colonial protozoa, the first differentiation of the members or cells of the colony takes place. One part of the cells is set apart to continue the task of reproduction, while the main body of the colony does not participate any longer in the function of generation. As a rule, the reproductive cells are divided into two kinds, and these two kinds of cells conjugate with each other. The conjugation takes place either between different members of the same colony or between members of different colonies of the same species.
The conjugating individuals are similar in the simpler kinds of colonies. In the higher forms the two conjugating cells are easily seen to be very different from each other. One kind of cells is large, spherical and inactive. They are for this reason called egg-cells. The other kind of cells is small, with ovoid head and tapering tail and free-swimming. For this reason they are called sperm cells.
When the differentiation between the reproductive cells into two kinds has taken place, they are called sex cells. The sex cells are usually situated in groups, and the groups are called sexual glands. The groups of sperm cells are called testicles and those of egg cells are called ovaries.