SKULL OF A HUMAN FETUS OF 40 MM. 345 



of the body over the arch processes the occipital scleromere 

 resembles those of the atlas and axis. 



In the 14 mm. stage of Levi chondrous paired centers for the 

 neural processes of the occipital vertebra arise, lateral to the 

 hypoglossal nerve roots, and speedily unite by continuity of 

 cartilage with the nodules just mentioned, which also join with 

 one another in the same way, at first ventrally. The fused 

 basilar centers are a little later joined by the chondrifying middle 

 part of the undifferentiated sclerotomes to form the median 

 part of the pars basilaris, and the chondrifying lateral masses 

 of the undifferentiated sclerotomes join the neural processes. 

 Thus the basilar portion, which may be taken to extend to the 

 ventral margin of the hypoglossal canal is built up from the 

 middle parts of all of the primitive occipital segments, the por- 

 tion included in the anterior margin of the foramen magnum 

 arising from the body mass of the occipital scleromere, and the 

 cranioventral part coming from the body masses of the undiffer- 

 entiated sclerotomes. 



A lateral outgrowth from the neural arch is seen in a mem- 

 branous condition in the 13 mm. stage of Levi, and is somewhat 

 later in chondrifying than the neural arch of either the occipital 

 vertebra or of the atlas. It represents, doubtless, the trans- 

 verse process of the occipital vertebra, and is spoken of by Levi 

 as the Querleiste. The costal process of the occipital vertebra 

 has not been shown to have a separate center of chondrification. 



It is to be noted that in the 13 mm. stage of Levi the bodies 

 of the occipital vertebra, atlas and axis are represented by paired 

 masses of chondrogenic mesenchyme, separated by the perichordal 

 septum, and each mass contains a small nodule of cartilage (except 

 in the case of the axis, where the two nodules have fused), their 

 neural arch processes being entirely membranous. In the 14 mm. 

 stage of Levi not only are the bodies chondrified but also the 

 neural arches; for in the occipital vertebra a chondrous center 

 appears just lateral to the hypoglossal foramen, and the arches 

 of the atlas and axis also present each a small nodule of carti- 

 lage. Thus chondrification takes place simultaneously in cor- 



