THE SHAPES OF VERTEBRATE SKELETONS. 201 



the evolution of the vertebrate axis, to find that as the mus- 

 cular power becomes greater there arise larger and harder 

 masses of tissue, serving the muscles as points cVappui ; and 

 that these arise first in those places where the strains are 

 greatest. ISTow this is just what we do find. The myocom- 

 mata are so placed that their actions are likely to affect first 

 that upper coat of the notochord, where there are found 

 " quadrate masses of somewhat denser tissue," which " seem 

 faintly to represent neural spines," even in the AmpJiioxus. 

 It is by the development of the neural spines, and after them 

 of the haemal spines, that the segments of the vertebral 

 column are first marked out ; and under the increasing strain 

 of more-developed myocommata, it is just these peripheral 

 appendages of the vertebral segments that must be most 

 subject to the forces which cause the formation of denser 

 tissue. It follows from the mechanical hypothesis that as 

 the muscular segmentation must begin externally and pro- 

 gress inwards, so, too, must the vertebral segmentation. 

 Besides thus finding reason for the fact that in fishes with 

 wholly cartilaginous skeletons, the vertebral segments are 

 indicated by these processes, while yet the notochord is un- 

 segmented ; we find a like reason for the fact that the tran- 

 sition from the less-dense cartilaginous skeleton to the more- 

 dense osseous skeleton, pursues a parallel course. In the 

 existing Lepidoziren, which by uniting certain piscine and 

 amphibian characters betrays its close alliance with primitive 

 types, the axial part of the vertebral column is unossified, 

 while there is ossification of the peripheral parts. Similarly 

 with numerous genera of fishes classed as palseozoic. The 

 fossil remains of them show that while the neural and haemal 

 spines consisted of bone, the central parts of the vertebrae 

 were not bony. It may in some cases be noted, too, both in 

 extant and in fossil forms, that while the ossification is com- 

 plete at the outer extremities of the spines it is incomplete 

 at their inner extremities — thus similarly implying centri- 

 petal development. 



