342 CHARLES CLIFFORD MACKLIN 



occipital incisure, and dorsally the modified cartilage may be 

 traced as far as my sections go. This area no doubt represents 

 the center of ossification of the supraoccipital, which according 

 to Mall ('06) ossifies from four centers, the first pair appearing 

 in the region immediately dorsal to the foramen magnum on the 

 55th to 56th day, and being followed by other paired centers in 

 the region ventral to this. 



In all there have been noted in the occipital region five com- 

 mencing centers of ossification, one, median, for the basilar 

 portion; two, lateral for the condyloid portions, and two, lateral, 

 for the squamous portions. In the vicinity of these centers the 

 cartilage grades off into that surrounding them. Of the rest 

 of the cartilage it may be noted that the material of the ventral 

 border of the paracondyloid process, of the ventral part of the 

 jugular tubercle, and of the body of the condyloid portion lateral 

 to the hypoglossal canal shows a more advanced character than 

 the remainder. 



The atlantooccipital capsules are present; each is a sac composed 

 of a dense sheet of perichondrium, covering the applied facets 

 of the atlas and condyloid portion of the occipital cartilage. 

 They are richly cellular, and above them the cartilage is of a 

 younger type than that found in the remainder of the mass, 

 but gradually grades off into the more mature type. 



The primitive foramen magnum (figs. 1-2) is, at this stage, 

 worthy of examination. It consists of three parts; a large 

 central area, with which are contjinuous ventral and dorsal in- 

 cisures. The central portion is bordered by the crescentic, 

 rounded neural arch, and its plane looks upward and slightly 

 forward. Ventrally is to be seen the intercondyloid incisure, 

 its plane looking dorso-cranially, and this plane forms an angle 

 with the main portion, which is open cranio-dorsally. This 

 incisure remains a part of the foramen, later undergoing widen- 

 ing, and rounding out at the tip, as may be seen by examining 

 the Hertwig model and the osseous skull. Behind the central 

 portion, and appearing as a dorsal prolongation of it, is the 

 superior occipital incisure, filled by the membrana atlanto- 

 occipitalis dorsalis. The plane of this incisure is somewhat 



