The petromastoid part is developed from four centers, which make their appearance in the cartilaginous ear capsule about the fifth or sixth month.

Their proximal or cranial ends are connected with the ear capsules, and their distal extremities are joined to one another at the symphysis by mesodermal tissue.

Several thousand species of fish have developed bony connections of some kind to link their swim bladders to the inner ear capsules, so that the vibrations picked up and amplified by the sympathetic resonance of the swim bladder are transmitted to the semicircular canals.