The relationship of the developing inner ear, subarcuate fossa and paraflocculus in the rat

A study of the development of the subarcuate fossa and its relationships to the semicircular canals and to the paraflocculus was made in the rat. Specimens ranging from 12‐day embryos through 31 days after birth and adults were used for dissection and microscopic studies. Two distinct steps are identified in the development of the fossa. First preceded by the differentiation of the semicircular canals, a primitive subarcuate fossa, which is completely filled with loose vascularized connective tissue, develops in the periotic capsule. Second, the primitive dura mater of the posterior cranial fossa evaginates into the area of the primitive fossa displacing the loose connective tissue contained therein, to form a laterally projecting cavity from the posterior cranial fossa, the definitive subarcuate fossa. Thus, the primitive subarcuate fossa appears passively as a product of the pattern of development of the periotic capsule. The paraflocculus of the cerebellum does not appear to influence the development of the definitive subarcuate fossa since it secondarily occupies the area of the fossa. The pedunculated nature of the paraflocculus is associated with, and probably results from, the diameter of the ostium of the fossa being smaller than its greatest width. A comparison is made of the development of the subarcuate fossa within the periotic capsule in the rat and man. In the adult rat a definitive subarcuate fossa is present, while in adult man only a rudimentary counterpart exists.