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. In the lower classes in the animal kingdom the male and female glands are situated at different parts of the same animal, or near each other. Sometimes the same gland is producing both kinds of cells, male and female. In the hermaphroditic species of round worms, for instance, it is found that, when the reproductive organ is fully formed, it functions first as a testicle. The germ-cells at the anterior end of the sexual gland begin to divide rapidly and become small spermatozoa which are stored up in a receptacle of the uterus. Later on other cells, also situated at the anterior end of the sex-gland, begin to grow larger, store up yolk and become large egg-cells. They then enter the uterus and become fertilized by their own spermatozoa. This mode of fertilization is the so-called self-fertilization. It is found in the slightly complex animals, as the tape-worm or the leech, where one and the same individual produces both egg-cells and sperm-cells. In colonial jelly-fishes certain members of the colony produce only sperm cells, and certain other members produce only egg-cells.