431 Royal Society : — 



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

 into the ha3mal arches. 



4. In some Vertehrata 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 o%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 : — 1. A diaphysial region immediately surrounding the noto- 

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

 enings. The paraphysial regions give rise to the cartilaginous neural 

 and hfemal semi-arcs, which are primitively continuous -nith 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 

 hajmal semi-arcs also tend to unite below, but in a somewhat differ- 

 ent 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. 



The distal, larger part of the haemal semi-arc is commonly di- 

 stinguished from its proximal, smaller part by the conversion of its 

 cartilage into osseous or other tissue, and thus the semi-arc becomes 

 separated into a rib and an articular surface or process, for the head 

 of that rib, to which last the term Parapophysis may be conveniently 

 restricted. 



In the dorsal vertebra} of many Vertehrata, the neural semi-arc 

 sends out a process, the Biapophysis, which is eventually met by a 

 corresponding outgrowth of the rib, its so-called tubercle, and the 

 two become firmly connected together. 



When ossification occurs, it is a very general, if not invariable rule, 

 that an annular deposit around the notochord takes place in the 

 centrum. I term this the Diaphysis of the vertebra. In some 

 fishes a distinct centre of ossification appears in each paraphysial 

 region, and this may be termed the Paraphysis of the vertebra. 



In mammals each end of the vertebra ossifies from a distinct 

 point, and constitutes a central Epiphysis of the vertebra ; and in 

 many Vertehrata a part of the under surface of a centrum ossifies 

 separately as a distinct Hypophysis. It is another very general, if not 

 invariable nUe, that a distinct centre of ossification appears in, or 



