If we turn to the plant kingdom, however, we find a comparatively common organism of somewhat similar form. This is the Volvox, a plant which consists of some thousands of cells, and reaches a size of about a pin-head. It has the form of a hollow sphere, the wall of which is one cell thick, and the cavity of which contains only water. The cells bear whip-like cilia on their outer surface, by whose means the organism is able to move, swimming by a rotary motion round a definite axis. The individual cells are separated by layers of a gelatinous substance, through which, however, pass connecting strands of protoplasm. The cells, of course, contain the green colouring matter common to plants in general. Distributed among the ordinary cells occur a few that are distinguished by their larger size, and by the fact that they lack cilia. These are the special reproductive members of the colony. When the Volvox reaches maturity, these cells begin to divide, and form new growths which take the form of hollow sacs, which project into the cavity of the parent sphere. Later they separate from the wall of the parent, and begin to move about, in the internal cavity, by means of the cilia which they have developed. Finally the parent breaks up and dies, and the progeny are set free to commence life for themselves. [Illustration: FIG. 28.--Volvox. A female, showing egg cells.] [Illustration: FIG. 29.--Volvox. Male, showing packets of sperm cells.] The fundamental importance of this type is that we have already a division of the life activities. The majority of the cells are concerned in the nutrition of the individual as a whole. These ultimately perish. A minority, however, are fed and protected by them, and these in return secure the perpetuation of the race. This division into a mortal 'body' portion and an immortal reproductive portion is the first and most important division of the life activities, whether in the animal or in the plant kingdoms. The body cells, modified in various directions for their special purposes, could not, and do not, reproduce complete new individuals. Therefore a generalised type of cell is maintained for the express purpose of the propagation of the race. It is to be observed, now, that the process of reproduction in Volvox is not always such as we have described. Sometimes the reproductive cells are of two kinds. The one type divides into a great number of small ciliated cells, which escape separately and directly to the _outside_ of the sphere, and swim away. These free-swimming individuals do not form new colonies, but seek out the reproductive cells of the other type, which latter still form part of the organism which has produced them. One of the free-swimming cells enters each of those of the other kind, and the nuclei of the two merge into one. The cell so produced, after a longer or shorter rest period, commences to divide and redivide in the manner already described, forming a new colony.