PRIMORDIAL CRANIUM OF THE CAT 369 



terius, and it is doubtful, notwithstanding the great space be- 

 tween these parts, if another center does develop and enter into 

 the formation of the parietal plate. In favor of this assumption 

 is the brief period between the stage when the orbito-parietal 

 commissure and tectum posterius are separate (15 mm.) and the 

 stage when they are joined (17 mm.). Apparently, in the cat, 

 the parietal plate is formed by the coalescence of two cartilages 

 arising independently, one, mostly anterior to the otic region, 

 which gives rise also to the larger part of the orbito-parietal com- 

 missure and to the parieto-capsular commissure; the other dorsad 

 of the interval between the lateral occipital arch and pars cana- 

 licularis which unites with these parts, forms the broad caudal 

 portion of the parietal plate and also gives rise to the tectum 

 posterius. 



Facial and acustic nerves. The suprafacial commissure, form- 

 ing the roof of the primary facial canal, separates the facial nerve 

 from the ganglion semilunare. As the seventh nerve (including 

 the pars intermedia Wrisbergii) leaver the canal, the geniculate 

 ganglion is formed on its dorsal side. This is in contact with 

 the ganghon of the trigeminus, both structures lying outside the 

 plane of the fenestra sphenoparie talis. The lateral opening of this 

 primary facial canal should be compared with the foramen faciale 

 of reptiles. A foramen or canal, traversed by the facial nerve 

 beyond the ganglion genicuh, is a new acquisition for mammals 

 and not to be found in the reptilian cranium. Such is the stretch 

 which Fischer ('01) has described in Talpa, roofed over by the 

 "ganz dlinne Knorpelspange" (p. 504), separating the proper 

 facial opening from the hiatus spurius. Also the foramen faciale 

 externum of the tegmen tympani of the rabbit forms an acquisi- 

 tion to the primary facial canal, whose lateral opening in the tym- 

 panum is the apertura tympanica. This conception of the facial 

 canal is partly in accord with that of Vrolik ('73). In the cat, 

 at the stage modeled, the exit of the primaiy facial canal is at the 

 level of the ganglion geniculi (position of the future bony hiatus 

 canalis facialis) and outside the cavity of the chondrocranium. 

 In the bony cranium of cat, and probably in later stages of the 

 chondrocranium, the exit from the cranial cavity is by the aper- 

 tura tympanica. 



