1. Bone formation in the rabbit kidney with ligated vessels takes place (a) through the activity of young fibroblasts which accumulate to form a membrane-like structure; (b) subsequently by direct ossification of hyaline connective tissue in continuity with preformed bone; and (c) through erosion of lime placques by granulating tissue and laying down of lamellar bone by cells derived from fibroblasts.

2. Bone formation in the rabbit kidney begins not in direct contact with calcium deposits, but in the loose vascular connective tissue close under the transitional epithelium of the calices.

3. With autotransplanted ear cartilage of the rabbit there is an active new formation of cartilage in the connective tissue which surrounds the transplants, and the bone is formed by the fibroblasts from the perichondrium which erode and invade the calcified areas in this new cartilage.

4. The process of bone formation in the kidney is similar to that found in normal membranous ossification, while with the transplanted ear cartilage the process is identical with endochondral ossification.

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