Elastic fibres may make their appearance in it to form elastic cartilage, as in the case of the epiglottis. In these various instances, although it is perfectly true that tissues are formed by cell division, the cells are not, strictly speaking, separate units. They are not completely divided one from another. It is impossible to recognize their anatomical boundaries. But there is a much more serious difficulty in applying the cell theory—the difficulty of deciding what are the essential parts of a cell. Long ago it was recognized that many animal cells—white blood-corpuscles, for example—have no cell-wall. It was therefore decided that cell-body and nucleus are the only essential parts. But what is to be said of the red blood-corpuscles of mammals? (Fig. 4). Are they cells? They have neither cell-walls nor nucleus; nor does their substance present the structure which is usually associated with the “body-substance” of cells. They are not produced, if the view held by many histologists be sound, by cell division, in the ordinary sense of the term, but appear as spots, gradually growing into discs inside the body of a blood-forming cell. The discs are extruded when they reach their full dimensions. Yet the tissue, blood, is composed of these blood-discs and the intermediate substance blood-plasm. Mammalian blood might be dismissed as a non-cellular fluid secretion containing formed elements, if it were not for its history. In all animals below mammals the red corpuscles are cells with nuclei and cell-bodies. The absence of nuclei in mammals is due to the recognition by Nature of the fact that, as the blood-cells will never be called upon to divide, it is a waste of material to provide each of them with a nucleus. Not only would the nucleus be useless, but it would take up space, diminishing the capacity of the corpuscle for carrying hæmoglobin. The process of cell division is in consequence curtailed. There are, it is true, other ways of looking at this problem. The cells which line the bloodvessels stand in some sort of nutritive relation with the blood. When the lining cells of the bloodvessels are injured or inflamed, the blood clots. But here again it is somewhat straining a point to say that these lining cells are the cells of the blood, and the blood a kind of intercellular substance; especially as a distinction would have to be made between mammals with non-nucleated blood-corpuscles and birds with complete blood-cells.