CHAPTER XIV 
THE SKELETON 
I. GENERAL 
From an embryological point of view, the bones of the body, 
their associated cartilages, the ligaments that unite them together 
in various ways, and the joints should be considered together, 
as they have a common origin from certain aggregations of 
mesenchyme. The main source of the latter is the series of 
sclerotomes, but most of the bones of the skull are derived from 
the unsegmented cephalic mesenchyme. 
Most of the bones of the body pass through three stages in 
their embryonic development: (1) a membranous or prechondral 
stage, (2) a cartilaginous stage, (8) the stage of ossification. 
Such bones are known as cartilage bones, for the reason that 
they are preformed in cartilage. Many (see p. 433 for list) of 
the bones of the skull, the clavicles and the uncinate processes of 
the ribs do not pass through the stage of cartilage, but ossifica- 
tion takes place directly in the membrane; these are known as 
membrane or covering bones. The ontogenetic stages of bone 
formation parallel the phylogenetic stages, membrane preceding 
cartilage, and the latter preceding bone in the taxonomic series. 
Thus, in Amphioxus, the skeleton (excluding the notochord) 
is membranous; in the lamprey eel it is partly membranous and 
partly cartilaginous; in the selachia it is mainly cartilaginous; in 
higher forms bone replaces cartilage to a greater or less degree. 
The comparative study of membrane bones indicates that they 
were primitively of dermal origin, and only secondarily grafted 
on to the underlying cartilage to strengthen it. Thus the ear- 
tilage bones belong to an older category than the membrane 
bones. 
The so-called membranous or prechondral stage of the skeleton 
is characterized simply by condensation of the mesenchyme. 
Such condensations arise at various times and places described 
407 
