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 Am=nzota 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 ammata 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 would be to institute a single centre of ossification, 
