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The embryologically unique origin of the temporomandibular joint from two separately (in time and space) developing blastemas was further studied in relation to temporal structures. Both coronal and sagittal sections of 11 human fetuses from the 24 mm to 72 mm crown-rump ( C-R) stage showed that the glenoid blastema developed in close relationship with the primordial quadrate-meckelian joint to form the bony temporal joint components, including the capsular elements of the upper joint story. The disk developed independently of the tendon of the external pterygoid muscle as a derivative of both the condylar and the glenoid blastemas.

Endochondral growth activity of the mandibular condyles, in contrast to epiphyseal plates of long bones, is responsive to induced stresses.1 2 This unique behavior of condylar cartilage is accompanied by peculiarities in phylogenesis, histogenesis, and metabolism which differ from the pattern of epiphyseal growth cartilages: (1) Phylogenetically, the temporomandibular joint represents a new acquisition of the order of mammals; it replaces the articulation of higher reptiles between the articulate and the quadrate, which, in mammals, become ear ossicles (malleus, incus). (2) Histogenetically, condylar cartilage accrues by surface apposition in a peripheral fibrocartilage, whereas epiphyseal plates proliferate interstitially. (3) Metabolically, epiphyseal cartilages show much greater respiratory activity than found in articular cartilages.3 The hormonal regulation of condylar development also differs from epiphyseal plates.4The view has been expressed that this unique position of condylar cartilage among endochondral growth centers may be explained on the basis of its specific embryonic origin.5 A review of the literature recently presented by R. P. Cousin6 reveals only scanty and contradictory information regarding the embryogenesis of condylar cartilage. Scott and Symons7-to our knowledge-are the only textbook authors to present authentic human material showing the first appearance of "secondary" condylar cartilage at the 48-mm. stage as a fringe on the dorsal end of the membranous dentale. This conforms with the observations made by Mundaca,

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