*e.* Each of the chromosomes split at full length. *f.* Beginning of division of protoplasm; the two parts of each chromosome separated. *g.* End of cell-division.] Each cell-division which promotes growth is followed by the enlargement of the two daughter-cells which result from it; these two daughter-elements attain the exact size of the mother-cell before division, and as soon as this size is reached a new division begins: so the growth of the whole is in the main the result of the growth of the elements. Cell-divisions during real organ-formation may behave differently, as will be described at a proper occasion. THE EGG: ITS MATURATION AND FERTILISATION We know that all the organs of an animal or plant consist of cells, and we know what acts a cell can perform. Now there is one very important organ in all living beings, which is devoted to reproduction. This organ, the so-called ovary in animals, is also built up of cells, and its single cells are called the eggs; the eggs originated by cell-division, and cell-division is to lead from them to the new adult. But, with a very few exceptions, the egg in the ovary is not able to accomplish its functions, unless certain typical events have occurred, some of which are of a merely preparatory kind, whilst the others are the actual stimulus for development. The preparatory ones are generally known under the name of “maturation.” The egg must be “mature,” in order that it may begin development, or even that it may be stimulated to it. Maturation consists of a rather complicated series of phenomena: later on we shall have occasion to mention, at least shortly, what happens in the protoplasm during its course; as to the nuclear changes during maturation it may be enough for our purposes to say, that there occur certain processes among the chromosomes, which lead to an extension of half of them in the form of two very small cells, the “directive cells” or “directive or polar bodies,” as they have been somewhat cautiously called. The ripe or mature egg is capable of being fertilised.