the “blood-worms” of Egypt, or the trypanosomes (the cause of “sleeping sickness”) of Equatorial Africa. Occasionally, in the rare disease lymphocythæmia leucocytes multiply exceedingly, not, apparently, in response to a call for their presence in large numbers, but in defiance of the needs of the economy, and with baneful results. To the indispensable services which wandering cells render, frequent reference will be made. In the present connection, and while we are searching for the principles of construction of the animal body, it would be desirable, if we could do so, to define the status of wandering cells. If they entered the body from without, they would be parasites of commensal type, intruders who share in the food and shelter of the body in return for service. But they do not enter from without. They are cells of the growing body which, detaching themselves from the cells which are forming tissues, assume a wandering life. They are not to be recognized in the embryo until development is considerably advanced. Their origin is far from clear, but histologists believe that, although they are not recognizable as wandering cells in the earliest stages of growth, they, or rather their parent cells, are set apart at a very early date. Probably they are not formed in the embryo proper, but in the “extra-embryonic area,” from which they emigrate into the embryo. In this sense they come in from outside. But, after all, the extra-embryonic area equally with the embryo is a product of the ovum. Looking at the body as a whole, we recognize a common life, a soul in Aristotle’s sense, which inhabits the framework of fixed tissues; and at the same time we see a multitude of independent cells, each an organism in itself, produced, like amœbæ, from similar independent cells by cell division, absorbing the body-fluids, consuming invading germs and fragments of decaying tissues, dying, disintegrating, in their turn absorbed. Wandering cells are autonomous in the largest sense.