218 MORPHOLOGICAL DEVELOPMENT. 



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

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

 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 strains 

 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 

 unsegmented; we find a like reason for the fact that the 

 transition from the less-dense cartilaginous skeleton to the 

 more-dense osseous skeleton, pursues a parallel course. In 

 the existing Lepidosiren, 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 palaeozoic. 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. 



§ 257. After these explanations the process of eventual 

 segmentation in the spinal axis itself, will be readily under- 

 stood. The original cartilaginous rod has to maintain longi- 

 tudinal rigidity while permitting lateral flexion. As fast as 

 it becomes definitely marked out, it will begin to concentrate 

 within itself a great part of those pressures and tensions 



