CHAPTER L. 



THE ENDOSKELETON 



By the term skeleton we mean all hard parts used for support and 

 protection outside of what has already been termed the integument. The 

 skeleton develops only from mesenchyme. It will be recalled that after 

 the mesoderm has divided into a somatic and a splanchnic layer, these 

 two layers together are called mesothelium to distinguish them from the 

 Tiesenchyme, which, while also lying in the segmentation cavity, devel- 

 ops as separate cells from both the mesothelium and the entoderm. Some 

 even believe that ectoderm has a part in its formation. 



When bone forms from cartilage the lime salts may be laid down 

 on the inner portion of the perichondrium and from there invade the 

 cartilage. This is called ossification by ectochondrostosis. 



In the other way in which bones form from cartilage, the osteoblasts 

 are formed from the more interior cells, and then with this group of 

 osteoblasts as an ossification center, ossification extends in all directions. 

 This latter method is known as entochondrostosis. 



Often the long bones increase by smaller bones forming and then 

 becoming attached to the ends of the long bones. Such joining is called 

 an epiphysis. 



If madder is fed to an animal the actual bone formation is colored. 

 This makes it possible to see just how the new bone is formed. The new 

 bone is laid down outside of that already grown, and with such growth 

 the "marrow cavity" becomes larger by a resorption of the bone which 

 has already formed. The osteoblasts are laid down in between the newly 

 forming layers of bone (Fig. 403). 



THE VERTEBRAL COLUMN 



We have already seen in our study of the embryology of the frog 

 and chick how the centra of the vertebrae are formed around the noto- 

 chord and that possibly some parts of the chorda remain as the inter- 

 vertebral discs. Here we are to study and compare the adult form in 

 the various groups. 



The most complete vertebrae may be found in the tails of some of 

 the lower vertebrates. Figure 404 shows a comparison of several types. 



It will be noticed that dorsally there is a neural arch, while ventrally 

 a similar outgrowth from the centrum is known as the haemal arch, 

 while the pointed end in each case is known as a spine. 



The haemal arch is quite incomplete or even entirely absent in the 

 regions anterior to the tail. 



In the higher vertebrates (Fig-. 401) there are articular processes. 



