We investigated the attachment of tendons to the skeleton (tubercula humeri, patella) in regard to the chondrogenesis and osteogenesis in these areas. Then we compared our findings with those in other sites of skeletogenesis. A metamorphosis, transformation, of structure and nature of cells and intercellular susbstances in correlation with another by feedback characterized the transition from perichondrium and periost or tendon to the skeleton. The initial form of cells are in the case of tendons without doubt the special form of fibrocytes, the tendocytes. We believe that this finding confirm the earlier expressed opinion fibrocytes of the periost with the same structure as tendocytes metamorphose to osteoblasts. The metamorphosis of cells and intercellular substances at tendon attachments occur like in other regions of skeletogenesis (periost, epiphysis) in relative clearly definable stages: tendon, Ansatzstruktur (attachment structure), Ansatzknorpel (attachment cartilage) as a part of apophysis or Einstrahlungsknochen (invading bone). In regard to histogenesis periost and attachment structure are homologous formations. The latter one may be a reticular structure of collagen fibers or a fibrocartilage of which the cells are surrounded from a polysaccharide capsule and a fiber capsule partly accompanied by needle like minerals. The osseous attachment is composed from parallel running fibrils. This structure forms a Grenzstreifen (border sector) with a varied number of needle like crystals along the fibrils. The amount of minerals are increased in the Grenzlinie (border line) to bone and subsequently the crystals disappear. The tendinous mineralisation is a temporary one. However, the fibrils get the Fibrillencharakter (nature of fibrills) of the fibrils of lamellar bone. Later on the bone minerals appear. It is doubtful that osteogenesis occur always in the same manner. However, the vague term ossification can only concern the bone specific metamorphosis of fibrils, never the widely distributed mineralisation. Therefore, skeletogenesis ist one of the metamorphosis stages from the embryonal (!) determinated skeletal mesenchyme to mature and finally to old and atrophying bone. This metamorphosis may be a process probably following a controlled program. A developmental stage releases the development of the next one. A feedback influences the organisation of the different parts, e.g. cells and intercellularsubstances. The increasing diversity of the system developed in the way of Selbstorganisation (self-reproduction). With the exhaustion of the program informations started the ageing.