THE SHAPES OF VERTEBRATE SKEI^ETOXS. 199 



thin, but more decided and comparatively distant fissures aa 



thev become thick. Fio^ 289, which is a 



. ... ^^^ 



horizontal longitudinal section, shows 



how these thickening layers will adjust "x \ ^\ \ \^\ 

 themselves on the convex and the con- 

 cave surfaces, supposing the fibres of .<i^=zf=z=s=i£l^'^,'' 

 winch they are composed to be oblique, 

 as their function requires ; and it is not difiicult to see that 

 when once definite divisions have been established, they will 

 advance inwards as the layers develop ; and will so produce 

 a series of muscular bundles. Here then we have something 

 like the myocommata which are traceable in the Aniphiuxu-^, 

 and are conspicuous in all superior fishes. 



^ 256. These speculative conceptions I have ventured to 

 present with the view of showing that the hypothesis of the 

 mechanical genesis of vertebrate structure, is not wholly at 

 fault when applied to the most rudimentary vertebrate ani- 

 mal. Lest it should be alleged that the question is begged 

 if we set out with a type which, like the Amphioxtis, already 

 displays segmentation throughout its muscular system, it 

 seemed needful to indicate conceivable modes in which there 

 may have been mechanically produced those leading traits 

 that distinguish the AmphtoxNs. It seemed needful to 

 assign an origin for the notochord : and to this we see a 

 clue in the differentiating effects of the transverse strain. It 

 seemed needful to account for the existence of muscular 

 divisions while yet there are no vertebral divisions ; and for 

 this, also, the transverse strain furnishes a feasible reason. 



But now, having sh.o\vn that the actions and reactions in- 

 volved by its mode of locomotion, are possible causes of those 

 rudimentar}^ structures which the simplest vertebrate animal 

 presents, let us return to the region of established fact, and 

 consider whether such actions and reactions as we actuallv 

 witness, are adequate causes of those observed difi'erentiations 

 and integrations which distinguish the more-developed ver* 



