PRIMORDIAL CRANIUM OF THE CAT 353 



completed, in the full term fetus, by connective tissue stretching 

 between the free extremity of the anterior bar and the neural 

 arch, anterior to the first spinal nerve. 



Observations on the sulcus in the atlantal neural arch for the 

 first spinal nerve in man have been made by Macalister ('93), 

 who gave the name post-glenoid tubercle to the process which 

 rises from the lateral mass and hmits the sulcus anteriorly. This 

 process was found to vary considerably in its extent, an ob- 

 servation easily verified in even a small series of specimens. 

 MacaUster also noted the presence of an independent bony center 

 in the ligament completing the atlantal foramen in the atlases of 

 young skeletons. The question, whether this represents the 

 typical mode of origin of the bony rod which, in man, completes 

 the atlantal foramen, remains for future enquiry. Separate 

 ossifications in the region of the neural arch of the atlas, be- 

 tween it and occipital, have several times been observed in man. 

 Trolard ('92) found, in two instances, in the posterior ligament 

 of the atlanto-occipital joint an osseous bar placed horizontally, 

 in one case nearly reaching the midplane. The possibility of 

 relation of these independent ossicles in man to the arch of the 

 atlas has, I believe, not been considered. It is well known that 

 an osseous element in the posterior occipito-atlantal ligament, 

 compared with the proatlas of reptiles and extinct amphibia, 

 has been described for Erinaceus (see Baur, '94). The possi- 

 bihty of the proatlas being a component of the atlas in a per- 

 sistent type was many years ago suggested by Osborn ('00). 

 The question of special interest which I wish to mention here 

 in regard to the post-glenoid tubercle, the bar of bone complet- 

 ing the atlantal foramen and the ossicles in the posterior occipito- 

 atlantal ligament, is whether these structures may not possibly 

 represent parts of one element which, in a primitive state, was 

 separate from, but closely related to the atlas. 



Occipito-atlantal articulation. The discovery by Fischer ('01) 

 of a single, horseshoe-shaped surface at the ventral margin of the 

 foramen magnum in embryos of Talpa, articulating with the ven- 

 tral arch of the atlas would seem to lessen the gap between 

 reptihan monoconydlic and mammalian dicondylic articulation. 



