=Conjugation.--=At longer or shorter intervals, however, we find that two individuals, on the disruption of the old colony, instead of continuing the routine of establishing new colonies through a series of cell divisions, very radically alter their behavior. They unite and fuse into a single larger individual. This process is called _conjugation_. We find it occurring even in some species of ameba. The conjugating cells in some colonies are alike in size and appearance, in others different. =Specialization of Sex-Cells.--=A beautiful sphere-shaped colony known as _Volvox_ is to be found occasionally in roadside pools. Depending on the species of _Volvox_ to which it belongs, the colony may be made up of from a few hundred to several thousand individuals arranged in a single layer about the fluid-filled center of the sphere and bound together by a clear jelly-like inter-cellular substance. Each individual cell also connects with its neighbors by means of thin threads of living matter. One of the largest species is _Volvox globator_, one edge of which is represented in Fig. 5, p. 27. Mutual pressure of the cells gives them a polygonal shape when viewed from the surface. Each cell, with a few exceptions to be noted immediately, bears two long flagella, whip-like structures which project out into the water. The lashing of these flagella gives the ball a rotary motion and thus it moves about. When the colony has reached its adult condition and is ready to reproduce itself, certain cells without flagella and somewhat larger than the ordinary cells become more rounded in outline and increase considerably in size through the acquisition of food materials. They are then known as egg cells or ova. Each ovum finally enters on a series of cell-divisions forming a mass of smaller and smaller cells which gradually assumes the form of a hollow sphere like the parent colony. The young colonies thus formed drop into the interior of the parent colony to escape later to the outside as independent swimming organisms when the old colony dies and disintegrates.