PRIMORDIAL CRANIUM OF THE CAT 313 



commissure, the alicochlear commissure and the basicochlear 

 commissure behind the carotid arterj^ are hkewise unstained; 

 the commissura orbito-parietahs is represented by a faintly 

 stained strip of cartilage independent of both the orbital plate 

 and otic capsule. The parietal plate is a triangular cartilage 

 surmounting the extremity of the occipital arch and joined to 

 it; it is unconnected with, though close to, the pars canalicularis. 

 Its medial and dorsal angle is the beginning of the tectum pos- 

 terius. Sections (H. E. C. ser. 400) show -the material of the 

 basal plate and floor of the sella turcica to be young cartilage. 

 The basivestibular, ahcochlear and basicochlear commissures 

 are composed of mesenchyma. The suprafacial commissure ap- 

 pears as a bar extending from the anterior pole of the pars canal- 

 icularis over the nervus faciahs to the anterior part of the coch- 

 lear capsule. It is composed of precartilage, easily distinguished 

 from that of the otic capsule by the more abundant, clear ground 

 substance between the nuclei. The relations of this commissure 

 are of high interest. Followed laterally across the facial nerve 

 it meets the pars canalicularis over the pronainentia utriculo- 

 ampullaris superior. It then passes without the smallest differ- 

 ence in degree of chondrificiation, without boundary of any sort, 

 into the commissura orbito-parietalis. Chondrification of these 

 commissures is quite uniform, and represents a state of histogene- 

 sis different from that of the otic capsule. Between the orbito- 

 parietal commissure and the otic capsule is a plane of mesen- 

 chymal tissue. The separation of the suprafacial commissure 

 and otic capsule is not so sharp. 



When now sections are followed in series from the spot where 

 the suprafacial commissure joins the pars cochlearis, toward the 

 Hiedian plane, an equally interesting connection becomes mani- 

 fest. It was shown above that in embryos of 17 mm. the coch- 

 lear end of the commissure became continuous with a stretch of 

 precartilage in the anterior part of the cochlear capsule, corre- 

 sponding in shape and state of development exactly with that of 

 the commissure itself. In the present stage this stretch again 

 appears and can be followed medialward and cephalad almost as 

 far as the point of union of the commissura alicochlearis with the 

 cochlear capsule. 



THE JOURNAL OF MORPHOLOGY, VOL. 29, NO. 2 



