At one time it was thought that the cells in the tissues of plants and animals could originate _de novo_ by a sort of precipitation of liquid matter. But it is now known that every cell has originated by the division of a pre-existing cell into two, the nucleus of the mother cell first dividing and then the rest of the cell. “Every cell originates by the fission of a preceding cell” is the law, and to that is added, “Every individual organism, plant or animal, itself originates from a single cell, the fertilised germ-cell.” These are two laws of fundamental importance in the study of living things. They are true of man as well as of the smallest worm; of the biggest tree as well as of the most insignificant moss or water-weed. When the fertilised egg-cell divides, and its progeny keep on dividing and growing in bulk by the conversion of nutriment into protoplasm, the dividing cells do not necessarily become entirely nipped off from one another. In large tracts of cells (or tissues) we often find that the neighbouring cells are connected to one another by excessively fine filaments of protoplasm. Only twenty years ago it was supposed, whilst the neighbouring cells were thus connected as a rule in animals, as well as being often connected to the finest nerve-filaments, yet that in plants the firm, box-like cases which surround the protoplasm—and when seen dried and empty by Robert Hook led him to introduce the word “cell” to describe them—form completely shut cases, so that the living protoplasm of each plant-cell is entirely cut off from its neighbour. This has now been found by improved methods of microscopic examination to be a mistake. The cell-wall in a great many plants, though so firm and cleanly cut in appearance, is yet perforated by fine threads of the cell protoplasm, so that each cell is in living communication with its neighbour. Thus, in plants as well as in animals, the individual cell-units form a more or less continuous whole of living matter, separated by dead, inert cell-walls and products of cell activity; but, nevertheless, connected in definite tracts and regions to one another by continuity of the living matter in the form of excessively fine threads.