they are directed towards a useful and unforeseen end. In the process of healing and repair of a wounded part of the body, a fluid, perfectly structureless substance, is secreted, or poured forth, from the parts about the wound. In this substance, cells arise and become abundant; so that the substance, at first structureless, becomes what is called cellular tissue. Then, by degrees, this structure transforms itself into vessels, tendons, nerves, bone, and membrane—into some or all of such parts—according to the circumstances of the case. In a case of broken bone, the two broken ends of the bone soften, the sharp edges thus disappearing. Then a soft substance is secreted, and this becomes at first gelatinous, often afterwards cartilaginous, and, finally, osseous or bony. But not only do these different kinds of substance—these distinct tissues—thus arise and develop themselves in this neutral or, as it is called, “undifferentiated” substance, but very complex structures, appropriately formed and nicely adjusted for the performance of complex functions, may also be developed. We see this in the production of admirably formed joints in parts which were at first devoid of anything of the kind. I may quote, as an example, the case of a railway guard, whose arm had been so injured that he had been compelled to have the elbow with its joint cut out, but who afterwards developed a new joint almost as good as the old one. In the uninjured condition the outer bone of the lower arm—the radius—ends above in a smooth-surfaced cup, which plays against part of the lower end of the bone of the upper arm, or humerus, while its side also plays against the side of the other bone of the lower arm, the ulna, with the interposition of a cartilaginous surface. The radius and ulna are united to the humerus by dense and strong membranes or ligaments, which pass between it and them, anteriorly, posteriorly, and on each side, and are attached to projecting processes, one on each side of the humerus. Such was the condition of the parts which were removed by the surgeon. Nine years after the operation the patient died, and Mr. Syme had the opportunity of dissecting the arm, which in the meantime had served the poor man perfectly well, he having been in the habit of swinging himself by it from one carriage to another, while the train was in motion, quite as easily and securely as with the other arm. On examination, Mr. Syme found that the amputated end of the radius had formed a fresh polished surface, and played both on the humerus and the ulna, a material something like cartilage being interposed. The ends of the bones of the forearm were locked in by two processes projecting downwards from the humerus, and also strong lateral and still stronger anterior and posterior ligaments again bound them fast to the last-named bone.[7] It would be easy to bring forward a number of more or less similar cases. The amount of reproduction of lost parts