PRIMORDIAL CRANIUM OF THE CAT 311 



Otic region in smaller embryos 



In cat embryos of 17 mm. (H. E. C. ser. 492 and van Wijhe 

 preparations, fig. 8) both pars canalicularis and pars cochlearis 

 are largely chondrified and stand in connection with each other. 

 At their margins they are united with the floor and walls of the 

 cranium by commissures between which openings are found. 

 The basal plate, sella turcica, parietal plate, orbito-parietal com- 

 missure and occipital wall, parts adjacent to the auditory cap- 

 sule, are all formed of cartilage. The cartilaginous structure of 

 the ventral portion of the occipital wall and that of the floor of 

 the cranium are further advanced in development than the car- 

 tilage in the remaining parts. 



In the pars canaUcularis, cartilage has formed upon its lateral 

 surface, anterior, posterior and dorsal margins, around and be- 

 tween the semicircular ducts; its medial surface is chondrified 

 about the circumference and crus commune. The prominentiae 

 utriculo-ampullaris superior and inferior present cartilaginous 

 walls, that of the last named being continuous ventrally with 

 the cochlear wall. A chondrified medial wall of the vestibule 

 forms the dorsal boundary of the future internal acustic meatus. 

 At the margins of the pars canaUcularis the sections show that 

 planes of prochondral tissue separate it from the parieto-capsular 

 commitesure in front and lateral occipital arch posteriorly. 



As shown by the sections through the cochlear capsule, pre- 

 cartilage is everywhere present excepting at the anterior and 

 posterior poles. At these two spots the cochlea is united with 

 the cartilage of the cranial base by cominissures, but between 

 the commissures it is separated from the basal cartilage by an 

 extensive basicochlear fissure filled with mesenchyma. The roof 

 of the cochlear capsule is considerably further developed than 

 the floor and is nearer the cartilaginous state toward the pars 

 canaUcularis than in the direction of the median plane. At the 

 posterior pole young cartilage can be followed from the promi- 

 nentia utriculo-ampullaris inferior medially and caudally into the 

 cochlea. This area lies between the fenestra acustica and the 

 foramen perilymphaticum ; from it the basivestibular commis- 



