364 ROBERT J. TERRY 



opening exists, while in general constant for mammals, shows an 

 interesting variation in the cat. In mammals these openings lie 

 approximately one above the other, so that a superior and an 

 inferior acustic foramen, for the vestibular and cochlear nerves 

 respectively can be spoken of. In the cat, at the stage of 23.1 

 mm. the vestibular nerve occupies the antero-lateral part of the 

 fissure; the cochlear nerve the medio-caudal end. The antero- 

 posterior order of the acustic rami recalls the condition in rep- 

 tiles. Although a common meatus for the two divisions of the 

 acustic nerve occurs in birds (Tonkoff, '00) this is separate 

 from the exit of the facial nerve, and therefore is not compar- 

 able wilji the internal acustic meatus of manamals, which in- 

 cludes separate passages for the vestibular and cochlear rami 

 and for the nervus facialis. The development of the internal 

 acustic meatus begins very early and seems to be a result, not 

 of a depression of the medial otic wall, but, as Voit remarks, 

 of the elevation of the surrounding cartilaginous parts. In the 

 cat, the elevations are made by the suprafacial commissure 

 anteriorly, the otic extremity of the basivestibular commissure 

 posteriorly and the prominentia utricularis dorsally and later- 

 ally. These three parts rise above the level of the cochlear 

 roof and form three sides of the meatus acusticus internus and 

 porus acusticus. The low, medial and ventral side is the roof 

 of the cochlea. Of these three elevations, that of the supra- 

 facial commissure is most prominent and is probably the chief 

 factor in determining the presence of the meatus. 



Foramen perilymphaticum. Since Fischer's ('03) description 

 of the derivation of the aquaeductus cochleae and fenestra 

 cochleae from the perilymphatic foramen, several observations 

 have been made by other investigators concerning this interest- 

 ing phenomenon. Fischer found in an embryo Semnopithecus 

 that the downward directed opening at the basal side of the ear 

 capsule was divided into two parts by a process which sprang 

 from the anterior margin of the opening and extended backward. 

 The larger lateral part looked out from the free outer surface 

 of the skull and was closed by thick membrane ; it was identified 

 as the fenestra cochleae. The medial smaller opening, on a 



