S8o ON THE THEORY OF THE VERTEBRATE SKULL 



that each somatome gives rise to (a) epiaxial muscles, {b) a nerve' 

 and its ganglion, {c) the blastema for a vertebral centrum and its. 

 neural and haemal arches, and {d) possibly hypaxial muscles ; while' 

 the metasomatome becomes for the greater part of its extent an 

 " intermuscular septum." 



It is unnecessary for my present purpose to trace out particularly 

 the development of any of these parts, except the centrum and its. 

 arches. 



The blastema, which is specially intended for these parts, appears,, 

 in a distinct form, first, in the paraphysial thickenings, and then 

 extends inwards above and below, so as gradually to enclose the 

 notochord in a sheath, while, externally, it passes in the posterior half 

 of each somatome, upwards into the neural arches, and downwards, 

 into the haemal arches. 



4. In some Vertebrata the spinal column never gets beyond this, 

 stage, nor even so far ; but for the present it will be well to confine 

 our attention to those which become completely ossified. In these 

 chondrification is the next step. The blastema of the centra and its 

 prolongations becomes converted into cartilage, but not continuously.. 

 On the contrary, at points corresponding with the intervals between 

 every pair of metasomatomes, or with the middle of each somatome,, 

 the cartilage is replaced by more or less fibrous tissue. As a conse- 

 quence, the cartilaginous sheath of the notochord is now divided into, 

 regular segments, which alternate with the somatomes, so that each 

 metasomatome abuts upon the middle of one of these cartilaginous, 

 vertebral centra. 



In every centrum it is necessary to distinguish three tracts or 

 regions: — I. A diaphysial region immediately surrounding the noto- 

 chord. 2. Two paraphysial regions lying in the paraphysial thick- 

 enings. The paraphysial regions give rise to the cartilaginous neural 

 and haemal semi-arcs, which are primitively continuous with them ; sO' 

 that all parts of the vertebra form one connected whole. 



The neural semi-arcs eventually unite in the middle line, and 

 ordinarily send a prolongation upwards from their junction. The 

 haemal semi-arcs also tend to unite below, but in a somewhat different 

 manner. 



5. The last step in the development of the vertebra is the dif- 

 ferentiation of its various parts from one another, and their final 

 metamorphosis into their adult form. The notochord, which primi- 

 tively traversed the centra and the intercentra (intervertebral liga- 

 ments, synovial membranes, or the like, between the centra), becomes 

 more or less completely obliterated. 



