350 ROBERT J. TERRY 



neural arch of an ordinary vertebra is largely overcome. The 

 lateral occipital arch presents the same relations to a centrum 

 as does the atlantal neural arch, and also the same relations to 

 a hypochordal arch as obtains between the neural and ventral 

 arches of the atlas. That the order of formation of the hypo- 

 chordal and lateral occipital arches varies somewhat in different 

 species is no obstacle to the interpretation of their equivalency 

 to vertebral structures; the work of Weiss ('01) and Levi ('08) 

 has shown that there is also great variability in the develop- 

 ment of the ventral arch of the atlas among mammals. 



The fundamental differences between occipital and vertebral 

 development, which have been so clearly indicated by Noor- 

 denbos, must, I think, be recognized; but if the interpretation of 

 Weiss be correct, that the occipital develops not as a typical 

 vertebra, but like a specialized vertebra, namely, the atlas, we 

 must admit that there is still evidence of vertebral structure 

 in the skull. This interpretation is in accord with the physio- 

 logical environment of the region: a transitional zone between a 

 rigid (cranial) and a movable (spinal) division of the axial skele- 

 ton. Regarded from this aspect, the structural conditions are 

 seen to change as we pass along the vertebral column toward the 

 head, the form of the vertebrae becoming less typical, due to 

 tendencies along certain definite lines (regression of centrum, 

 development of hypochordal parts) . The occipital region shows 

 by its structures the culmination of these tendencies; extremely 

 rudimentary centrum separated from all connection with highly 

 perfected arches, lateral and hypochordal; characters which, to a 

 lesser extent, mark the atlas from a typical vertebra. 



There is probably more than a superficial resemblance be- 

 tween the lateral occipital arch and neural atlantal arch in re- 

 spect to the relations of these parts to nerves. The primary 

 notch formed for the hypoglossal nerve bundles at the side of the 

 parachordal plate, in front of the base of the lateral occipital arch 

 (Noordenbos describes it in the occipital arch), and its subse- 

 quent conversion into a foramen, are phenomena which seem to 

 parallel the development of the atlas in the region of the first 

 spinal nerve. In cat embryos of 15 and 23 mm. the first spinal 



