*Disappearance and Origin of Red Corpuscles.*—The red corpuscles, being cells without nuclei, are necessarily short-lived. It has been estimated that during a period of one to two months, all the red corpuscles in the body at a given time will have disappeared and their places taken by new ones. The origin of new corpuscles, however, and the manner of ridding the blood of old ones are problems that are not as yet fully solved. The removal of the products of broken down corpuscles is supposed to take place both in the liver and in the spleen.(9) Regarding the origin of the red corpuscles, the evidence now seems conclusive that large numbers of them are formed in the red marrow of the bones. The red marrow is located in what is known as the spongy substance of the bones (Chapter XIV) and consists, to a large extent, of cells somewhat like the red corpuscles, but differing from them in having nuclei. These appear to be constantly in a state of reproduction. The blood, flowing through the minute cavities containing these cells, carries those that have been loosened out into the blood stream. Nuclei appear in the red corpuscles at the time of their formation, but these quickly separate and, according to some authorities, form the blood platelets. *White Corpuscles.*—The white corpuscles, or _leucocytes_, are cells of a general spherical shape, each containing one, two, or more nuclei. They are much less numerous than the red, there being on the average only one white corpuscle to about every five hundred of the red ones. On the other hand, the white corpuscles are larger than the red, one of the former being equal in volume to about three of the latter. [Fig. 10] Fig. 10—*Escape of white corpuscles from a small blood vessel* (Hall). At _A_ the conditions are normal, but at _B_ some excitation in the surrounding tissue leads to a migration of corpuscles. 1, 2, and 3 show different stages of the passage. The white corpuscles are found, when studied under favorable conditions, to possess the power of changing their shape and, by this means, of moving from place to place. This property enables them to penetrate the walls of capillaries and to pass with the lymph in between the cells of the tissues. The white corpuscles are, therefore, not confined to the blood vessels, as are the red corpuscles, but migrate through the intercellular spaces (Fig. 10). If any part of the body becomes inflamed, the white corpuscles collect there in large numbers; and, on breaking down, they form most of the white portion of the sore, called the _pus_. New white corpuscles are formed from old ones, by cell-division. Their production may occur in almost any part of the body, but usually takes place in the lymphatic glands (Chapter VI) and in the spleen, where conditions for their development are especially favorable. In these places they are found in great abundance and in various stages of development.