186 Charles Eiissell Bardeen 



The articulation between the lateral mass of the atlas and the 

 superior articular surface of the epistropheus seems to be formed rather 

 in the interventral than, as in the other intervertebral diathroses, in the 

 interdorsal membranes. This is also true of the atlanto-occipital dia- 

 throsis. For a brief period (14 mm. embr3'o) the bases of the neural 

 arches of the atlas and epistropheus, together with the tissue inter- 

 vening between the bases of arches of the atlas and the occipital, become 

 fused into a nearly continuous mass of precartilage (Fig. 3). 



Hagen, His' Archiv, 1900, gives a somewhat different account of the 

 development of the atlas and epistropheus in man. He concludes (1) 

 that the dens epistrophei arises from the region of the body of the 

 epistropheus and a portion of the body of the atlas; (2) that the massse 

 lat. of the definite atlas arise from the rest of the primary anlage of 

 the body of the atlas, and (3) that the short piece which unites them in 

 front arises from the fusion of both neighboring septa. 



Basi-occipital. — Opposite the last occipital myotome the axial mesen- 

 chyme is differentiated, like that of the spinal sclerotomes, into a light 

 anterior half and a dense posterior half or scleromere. In the spinal 

 region each scleromere joins with the light half of the sclerotome next 

 posterior in giving rise to the body and arch processes of a spinal 

 vertebra. In man the occipital scleromere is not thus associated with 

 the light half of the first spinal sclerotome. On the contrary, it becomes 

 associated with the lighter tissue of its own segment and with the tissue 

 into which this is continued anteriorly. Laterally the tissue differen- 

 tiated at the side of the anterior half of the first spinal sclerotome, the 

 interventral membrane, becomes temporarily converted into a precar- 

 tilaginous membrane (Fig. 3). 



Chondrification of the base of the occipital begins in two bilaterally 

 situated centers in the posterior portion of the occipital anlage. The 

 union of these centers takes place posteriorly ventral to the notochord 

 and anteriorly dorsal to the notochord (Fig. 2). The neural processes 

 of the posterior part of the occipital anlage seems to have separate 

 centers of chondrification, but these centers fuse almost immediately 

 with the centers of chondrification of the body. 



From the tissue derived from the first and second sclerotomes and 

 not utilized in the formation of the atlas and the epistropheus are 

 derived the various ligaments which unite these bones. The details of 

 the formation of these ligaments are too complex for description here. 



A . 



