344 CHARLES CLIFFORD MACKLIN 



marked similarity to the vertebrae the occipital scleromere shows 

 certain individual peculiarities. Instead of uniting with the 

 cranial portion of the segment immediately caudal to it, after 

 the fashion of the other scleromeres, it retains its connection 

 with the cranial portion of its own segment, and this, in turn, 

 becomes united with the tissue of the three cranial, undifferen- 

 tiated sclerotomes, the membranous anlage thus formed being 

 known as the occipital plate (Bardeen '08 and '10). According 

 to Froriep the occipital scleromere is marked off from the un- 

 differentiated sclerotomes by the caudal root of the hypoglossal 

 nerve. 



The middlepiece of the occipital plate is made up in its caudal 

 portion of the chordal processes of the occipital scleromere, and 

 in its cranial part it also contains the elements of the body masses 

 of the undifferentiated segments. So also while the lateral 

 portions are mainly formed of the neural processes of the occipi- 

 tal scleromere they also contain, in the region cranial to the 

 hypoglossal canal, remnants of the condensed lateral masses 

 of the undifferentiated segments (Froriep, Levi). 



In the chondrogenous stage of the occipital anlage of man, 

 for the knowledge of which we are principally indebted to Levi, 

 there are also striking resemblances to the vertebral conditions. 

 The 13 mm. stage, studied by this author, shows the beginning 

 of the transition from the membranous to the cartilaginous 

 condition and in this paired masses of condensed chondrogenic 

 mesenchyme, separated by the perichordal septum, were situated 

 dorso-caudally in the middlepiece or basilar portion, and in the 

 dorsal part of each mass, medial to the hypoglossal nerve, a small 

 cartilaginous nodule occurred, recalling the paired chondrous 

 centers of the body of a spinal vertebra. The mesenchymatous 

 masses, representing the chordal processes of the occipital sclero- 

 mere, were joined ventrally ; they are the first portions of the occip- 

 ital anlage, and indeed of the entire chondrocranium, to under- 

 go chondrification, and eventually form the diverging, caudo- 

 lateral portions of the pars basilaris bordering the foramen 

 magnum. In the matter of priority in time of chondrification 



