PRIMORDIAL CRANIUM OF THE CAT 297 



vestibular commissure. Second, the anterior expanded ex- 

 tremity of the basal plate is united with the hypophyseal car- 

 tilage forming the crista transversa. The carotid artery as it 

 enters the cranium is pressed against the side of the expanded 

 end. 



The dorsal extremity of the lateral occipital arch is not. 

 united anteriorly witfi the pars canalicularis of the otic capsule; 

 it is, however, joined with the parietal plate which now pre- 

 sents a prominent dorso-medial angle, the beginning of the 

 tectum posterius. 



In the atlas the hypochordal arch is partly chondrified. It is 

 composed of two bars, right and left, which are continuous at 

 each side with the lateral masses and approach each other 

 medially. The lateral mass has been formed from the expanded 

 end of the hypochordal arch of the preceding stage. The 

 centrum of the atlas has united with that of the epistropheus so 

 that a dens epistrophei can be described. The anterior ex- 

 tremity of the latter projects cephalad beyond the transverse 

 plane of the atlantal hypochordal arch. The lateral arches of 

 the epistropheus are connected with the centrum by a lightly 

 stained zone. 



Sagittal sections of the embryos of 15 mm. show that the 

 basal plate is chondrified and is continuous with the floor of the 

 cranium in the hypophyseal region (fig. 18). Here, the anterior 

 end of the plate is still recognizable as an elevation, the crista 

 transversa, behind the hypophysis. The bent extremity of the 

 notochord lies upon the transverse crest imbedded in a hill of 

 mesenchyma. In the caudal region of the basal plate the noto- 

 chord still occupies a position near the intracranial surface, 

 covered by connective tissue continuous with the dens epis- 

 trophei. A superficial constriction marks the original plane of 

 fusion of the centra of the first and second vertebrae. Chon- 

 drification of the dens is less advanced toward its extremity than 

 at its base. Since the last stage described chondrification has 

 extended forward in the mesenchyma about the notochord, in 

 consequence of which the extremity of the dens has been length- 

 ened and now lies upon the caudal margin of the basal plate. 



JOURNAL OF MORPHOLOGY, VOL. 29, NO. 2 



