With regard to the former, we have seen that in the single cells, constituting unicellular organisms, there is always a bounding membrane of denser texture than the rest of the protoplasm. As the cell develops its capabilities, we have a shell or case of non-living matter secreted around it, with apertures for communication with the outside world, and increasingly effective protection is provided as protoplasm, whether in the single cell or the body, leaves the water, and has to face the inclemencies of terrestrial life. [Illustration: DIAGRAM 48.—SHOWING THE FORMATION OF THE SKIN.] [Illustration: DIAGRAM 49.—STRUCTURE OF SKIN.] In the schematic embryo (Diagram 6) and other diagrams contained in this volume, the skin has so far been represented as consisting of a single layer of living cells; but we must now admit that the skin of man is quite different. Such a covering would be no protection from heat, cold, or irritating chemicals, while, in order to prevent its drying up, it would have to be kept moist with slime, and we should look very like frogs. In order that an adequate defence may be provided for the body, this layer of cells divides tangentially, forming two layers. The inner of these two then divides tangentially again, and a second layer is interposed between the innermost and that first formed. The skin now consists of three layers, and so the process is repeated until it is several layers thick. (See Diagram 48.) It is the innermost and best-nourished layer which keeps dividing; the other layers, as they get pushed outwards, are only reached by a little lymph which filters between the cells, and are eventually starved even of that. As they get pushed away from the dividing layer, however, they set to work to surround themselves with a horny wall, which thickens and thickens, until eventually there is hardly any cell left. (See Diagram 49.) Finally the cells die and the horny envelopes form a dead cuticle, protecting the living layers beneath, and are ultimately sloughed off when their successors are ready to replace them. [Illustration: DIAGRAM 50.—SHOWING THE DEVELOPMENT OF HAIR.]