PRIMORDIAL CRANIUM OF THE CAT 397 



8. The upright position of the axis of the otic capsules in 

 cat embryos of the stage modeled is exceptional for mammals. 

 There is evidence in the literature that the obliquity of the otic 

 axis characteristic of the mammahan chondrocranium is attained 

 at somewhat different periods of development in different 

 species. 



9. The origin of the cartilaginous pars canahcularis of the 

 otic capsule is independent of other parts of the cranium. The 

 pars cochlearis arises in connection with the pars canahcularis 

 and the suprafacial commissure. The latter is continuous with 

 the orbito-parietal commissure, and is therefore, in part at 

 least, to be regarded as a parietal structure. The cochlear cap- 

 sule forms independently of the basal plate, which in the cat is 

 reduced to a narrow bar in the otic region. The observations 

 on the relationship of the cochlear capsule to the basal plate 

 and suprafacial commissure are interpreted to support Gaupp's 

 theory on the development of the cochlear capsule and reforma- 

 tion of the cranial base. 



10. A single acustic fissure, transmitting the two divisions 

 of the acustic nerve, is situated at the bottom of a shallow inter- 

 nal acustic meatus. The latter results, principally, from the 

 elevations of the suprafacial commissure and commissura basi- 

 vestibularis, these forming its anterior and posterior walls. 



11. The fenestra cochleae and aquaeductus cochleae result 

 from the division of the foramen perilymphaticum by a process 

 extending from the cochlear capsule. 



12. The cavum vestibulare of the cartilaginous otic capsule 

 of the cat is comparable with that of the reptile (Lacerta) in 

 possessing two subdivisions, a separating partition and similar 

 relations to parts of the membranous labyrinth. 



13. The lamina parietalis seems to be derived from two inde- 

 pendently arising centers of chondrification, an anterior one, 

 which also constitutes the orbito-parietal commissure and 

 unites with the otic capsule in the parietocapsular commissure ; 

 and a posterior center which unites, first, with the summit of 

 the lateral occipital arch and, next, with the pars canahcularis. 

 From the posterior center (called parietal plate in the descrip- 



