PRIMORDIAL CRANIUM OF THE CAT 363 



extent that the foramen for the facial nerve stands between a 

 sense capsule (otic) and cranial wall (basal plate). 



In accordance with our observations, the cochlear capsule in 

 the cat is developed in connection with the pars canalicularis 

 and with the suprafacial commissure which gives evidence of 

 being, in part at least, a parietal structure. The cochlear cap- 

 sulfe chondrifies independently of the basal plate, with which it is 

 secondarily united by commissures. Its precocious growth and 

 great bulk encroach upon the broad region occupied in lower 

 vertebrates by the basal plate. If we can, on the evidence given, 

 interpret the suprafacial commissure as a parietal structure in 

 the cat, it would appear that its relation to the cochlear capsule 

 (continuity) affords support to the theory (Gaupp) of the latter 

 having preempted the territory of the basal plate and developed 

 at its expense. 



Foramina aciistica and meatus acusticus internus. The acustic 

 foramen or fissure and the entrance to the facial canal are estab- 

 lished early in the development of the otic capsule. These 

 openings he between the suprafacial commissure behind and, 

 in the 23.1 mm. stage, are found at the bottom of a shallow in- 

 ternal acustic meatus. The single acustic fissure is constricted 

 in its middle so as partly to separate two wide divisions accommo- 

 dating the vestibular and cochlear nerves. Completely sepa- 

 rated foramina were not observed in any of the stages studied. 

 Comparable with, this form of acustic nerve entrance is the 

 single dumb-bell shaped foramen which IVIead has described in 

 Sus. In the rabbit (Voit) and the dog (Olmstead) there are 

 separate foramina for the vestibular and cochlear nerves. In 

 the cranium of Talpa of the stage described by Fischer there are 

 also two separate acustic foramina; but in younger embryos 

 Noordenbos found a round porus acusticus internus, in the 

 bottom of which openings for the cochlear and vestibular nerves 

 were not clearly separated on account of lack of chondrification. 

 The persistence of a single opening in the cat speaks for tardy 

 development of the medial wall of the ear capsule, a tendency 

 characteristic of the lower vertebrates. The relative positions 

 of the foramina, one to the other, or of the nerves, where a single 



