350 CHARLES CLIFFORD MACKLIN 



series of ossification centers presents itself, the uppermost member 

 of the series being found in the exoccipital, in the position in 

 which I have described it in my model, and being followed cau- 

 dally by the ossification centers for the 1st, 2nd, 3rd and 4th 

 cervical vertebrae; each center being uniformly situated with 

 reference to its respective arch. In the basi-occipital, too, the 

 single median ossification center recalls the center of a typical 

 vertebral body, and indeed the osseous elements of the occipi- 

 tal bone at birth are strikingly similar to those of a vertebra, 

 especially the fifth lumbar vertebra. 



Keeping in mind the intimate and peculiar relationship 

 which a typical vertebral arch bears to the cord which it encloses 

 it seems reasonable to suppose that this relationship would be 

 retained even after the vertebra had been taken up into the 

 skull. It seems much more reasonable, even on purely theoreti- 

 cal grounds, to assume that the arch of the occipital vertebra 

 goes to form the margin of the foramen magnum of the mature 

 skull than to postulate that it forms the jugular process, as 

 Levi maintains, and when it is realized that the latter is formed 

 from the transverse process of the occipital vertebra, and that 

 the margin of the foramen magnum is merely the modified arch 

 of the occipital vertebra the proper relationship of the parts 

 becomes intelligible. 



Not only do the neural arches of the occipital vertebra exhibit 

 a striking resemblance to those of the cervical vertebrae in each 

 stage which has been studied, but their behavior in growth re- 

 calls very strongly that of the arches below. This parallelism in 

 manner and time of development between the arch of the occipital 

 vertebra and those of the cervical vertebrae is. clearly shown 

 by an examination of the 13 mm., 14 mm., 17 mm., and 28 mm. 

 stages of Levi, my own 40 mm. stage, and the 80 mm. stage of 

 Hertwig. The neural arches of the occipital vertebra, small at 

 first, are seen to grow backwards and outwards, and then to 

 come together medially and dorsally, thus hedging in the dorsal 

 part of the foramen magimm, this process being duplicated 

 coincidently by each of the upper cervical vertebral arches. 



