The influence of the otic capsule in ambystomid skull formation.

In the absence of the otic capsule, or in the presence of capsular materials of varying volume or placement, the bones and cartilages of the contiguous skull showed several sorts of responses. A. The parietal and squamosal bones, and possibly the parasphenoid bone and parachordal cartilage apparently used the capsule as a substrate, spreading over its surface. B. The parietal, squamosal, and exoccipital bones, and the quadrate cartilage were displaced when otic capsule material was absent or oversized. C. The squamosal bone developed at first independently of the capsule but was modified in its shape and size by the capsule in later, possibly inductive, response. D. Stresses resulting from paired otic capsules of unequal size bent the parasphenoid bone and the parachordal cartilage through angles of predictable direction relative to the notochord. E. The paired exoccipital bones developed at different rates when one otic capsule was absent or oversized. The results obtained following manipulation of the otocyst indicate the major role of extrinsic (epigenetic) parameters in normal skeletogenesis and emphasize an apparent discrepancy between the normal and potential expansion of a bone.