SKULL OF A HUMAN FETUS OF 40 MM. 343 



steeper than that of the part in front, and thus it forms an angle, 

 looking cranially and slightly ventrally, with that of the main 

 area. This is shown upon the lateral contour of the skull as the 

 point of an angle, directed downward and backward, marking 

 the tip of the dorsal foraminal prominence (fig. 3). 



The superior occipital incisure, though representing the dorsal 

 part of the primitive foramen magnum, is by no means a part 

 of the adult foramen magnum, as its edges unite later to com- 

 plete the caudo-dorsal closure of the occipital region, the dorsal 

 limit of the foramen magnum becoming set by the approxima- 

 tion of the dorsal foraminal prominences, as shown by the 

 researches of Bolk ('04). 



I find no evidences of the condition which Bolk describes at 

 the posterior extremity of the foramen magnum, viz., a central 

 cartilaginous mass (formed by the fusion of paired pre-existing 

 masses) which lies between the upturned dorsal extremities of 

 the occipital side-walls. 



It seems clear from the work of various investigators, begin- 

 ning with Froriep ('86), that although the occipital region of the 

 mammals has been developed from the skeletogenous elements 

 of four metameres, only the most caudal ever attains the status 

 of a mature sclerotome, the three cranialmost being undiffer- 

 entiated and playing but a minor part in the construction of the 

 adult bone. This being true it follows that the story of the 

 evolution of the occipital anlage is largely the story of the develop- 

 ment of the caudal segment, or, as it will be hereafter called, the 

 occipital scleromere or primitive occipital vertebra. 



As in the spinal region, so in the occipital, the sclerogenous 

 tissue passes through successive and overlapping membranous 

 or blastemal, chondrogenous and osteogenous phases (Bardeen 

 '05, '08), and in each phase the condition in the occiput recalls 

 that of the corresponding phase in the vertebrae. Thus in the 

 blastemal stage the occipital scleromere shows paired chordal 

 processes joining across the midline in the region of the notochord, 

 and paired neural processes embracing the neural canal as in the 

 spinal scleromere; the costal processes are, apparently, poorly 

 developed in the occipital scleromere. But along with this 



