of the Vertebrate Skeleton. 31 



constantly undergoing change. Both hjemal and neural parts 

 terminate in the head, but under these different conditions — 

 that while the neural arch is being modified for the first time, 

 the htemal arch undergoes its second transformation, which 

 may be altered to some extent by the relation of the two 

 arches to each other ; so that, on h priori grounds, the hajmal 

 arch in the skull may be expected to be more complex than 

 the neural arch, and also to more readily assimilate to the 

 haemal arches of the body. 



Now, if the brain-substance is supposed to have accumulated 

 at the anterior end of the body as a consequence of the motion 

 and mode of growth of the animal, and quite irrespective of the 

 vertebrae, its covering from the very first experienced some 

 different conditions of ossification from those of the vertebral 

 neural arch — supposing, of course, an anterior enlargement of 

 the nervous system to have taken place prior to the entire 

 segmentation of the notochord. Such a view, however, is not 

 supported by the evidence from Amphioxus^ since the noto- 

 chord is segmented and no brain developed. And the difficulty 

 of a theory of the skull hinges upon the relative probability 

 of the skull originating prior or subsequently to segmentation 

 of the notochord — because in the one case it will be but an 

 extension onward of the vertebral plan, and in the other case 

 it may have originated apart from the vertebral basis. If the 

 Amphioxus is a distinct type animal from the Vertebrata, 

 we shall not be warranted in reasoning from it to a vertebrate. 

 But, whatever the initial circumstances were which governed 

 the formation of a brain-case, we shall not be justified, except 

 with good evidence, in assuming any other cause to account 

 for it than potential repetition, which under altered conditions 

 has been found competent to produce very different osseous 

 structures in different p^rts of the vertebral column, especially 

 as the brain offers a surface to be covered different from the 

 spinal cord, and conditions of stability different from the visceral 

 region. It has been seen, with the diverging vertebral pro- 

 cesses, that, under the new conditions, osseous elements come 

 into existence which were not found in the caudal region : 

 similarly it will not be surprising if some new structures are 

 developed in the head by the special influences working in 

 that part of the body. 



Suggestive evidence of original unity of origin, direct or 

 indirect, for the whole skeleton, is supplied by the skull being 

 segmented, as it is shown to be by well-made researches ; for 

 if it had originated independently, no trace of segments could 

 be anticipated, but an arrangement of bones with which the 

 spinal column would have at first nothing in common, though 



