TMJ development starts at:

Three phases in the development of the TMJ were identified: The first is the blastematic stage (weeks 7-8 of development), which corresponds with the onset of the organization of the condyle and the articular disc and capsule. During week 8, intramembranous ossification of the temporal squamous bone begins.  The second stage is the cavitation stage (weeks 9-11 of development), corresponding to the initial formation of the inferior joint cavity (week 9) and the start condylar chondrogenesis. Week 11 marks the initiation of organization of the superior joint cavity.  And the third stage is the maturation stage (after week 12 of development).