360 ROBERT J. TERRY 



cartilage first appearing in the pars canalicularis from what has 

 been observed in other animals. I refer to the observation of a 

 plate of cartilage upon the lateral side of the semicircular canals; 

 in the cat, cartilage forms upon the lateral surfaces of the canals 

 in more or less separate stretches for each canal. The canals 

 are subsequently completely walled and the intervals between 

 them filled through the development of cartilage, but whether 

 this is by extension of cartilage formation from the canalicular 

 walls already established or from independent chondrifying 

 centers was not observed. The more or less compact mass con- 

 stituting the pars canalicularis is secondarily connected with 

 the lateral occipital arch and parietal plate, although its dorsal 

 margin remains free in the fissura jugulare spurium, and the 

 posterior margin is clearly indicated, even at the 23.1 mm. 

 stage, in the stretch of young cartilage between it and the lateral 

 occipital arch directed toward the jugular foramen. The pars 

 canalicularis is also apparently formed independently of the 

 suprafacial commissure, if one may judge this by the difference 

 in degree of development of these two closely associated parts. 

 In the pars cochlearis, cartilage was first observed in the 

 region next the pars canalicularis and in the anterior and pos- 

 terior poles, i.e. in the neighborhood of the suprafacial and basi- 

 vestibular commissures. There was no actual separation of these 

 chondrifying tracts from one another, no independent centers of 

 cartilage formation. At the stage when the medial wall of the 

 cochlear capsule is in precartilage, the capsule is separated 

 from the chondrified basal plate by a fissure filled with mesen- 

 chyma. Union of the capsule by cartilage with the basal plate 

 behind the carotid foramen, with the alicochlear commissure and 

 basivestibular commissure, is brought about secondarily. Be- 

 tween the connnissura suprafacialis and anterior pole of the 

 cochlear capsule a smaller degree of difference in development 

 obtains than is the case between this commissure and the pars 

 canahcularis ; the ventral end of the commissure, however, 

 appears to be blended, if not actually continuous, with the coch- 

 lear capsule. In this discussion, the fact of the continuity of 

 these two parts is important because of the possibility of pa- 



