VERTEBRATA. 413 



individuals. These are the membranous, the cartilaginous 

 and the bony. All arise from the mesoblast : the first is 

 continuous, the second is largely segmented and the third is 

 completely segmented. 



In Myxine we find the membranous skeleton enveloping 

 the notochord or primary chordate skeletal axis and the 

 nerve - cord, and continued into the septa between the 

 myomeres. There is little progress here beyond Amphioxus. 



Cartilaginous nodules in the vertebral column, a cartila- 

 ginous cranium, and other parts appear in the lampreys, and 

 a more or less complete cartilaginous skeleton is found in 

 the skate. 



In bony fishes and in the Amniota the cartilage becomes 

 supplemented and eventually replaced by a bony skeleton. 



Bone is produced by the secretory activity of certain 

 cells called osteoblasts, and bones are known as membrane- 

 bones or cartilage-bones, according to their origin. The 

 membrane-bone is produced at once in the connective or 

 membranous tissue, whereas the cartilage-bone is preceded 

 by cartilage which has to be removed piecemeal as the bone 

 is produced. The distinction is merely arbitrary, and is 

 somewhat the same as the difference between building a 

 roof with single slates in situ (cartilage- bone) and construct- 

 ing an entire roof (as do many primitive peoples at the 

 present day), and then lifting it into position (membrane- 

 bone). The latter is, in each case, the more primitive 

 method. The final result in each kind of bone is the same, 

 and the two kinds cannot be structurally distinguished. 



Complete ossification is usually effected fairly late in 

 life, mainly because cartilage can grow more readily than 

 bone. In nearly all the Mammalia most of the bones have 

 separate caps or epiphyses at each end, probably to allow 

 free use of a formed joint in the early stages, whilst the 

 parts between the epiphyses and the main bone are still 

 growing cartilage. In late life the epiphyses usually fuse on 

 to the main bone. 



The replacement of cartilage by bone is effected from 

 certain centres, called centres of ossification, and the history 

 of these throws light upon many obscure points in the 

 skeletal structure. The simplest plan for -the ossification of a 

 long bone w ould be to institute a single centre of ossification, 



