ANATOMY 



by surface-cells. The shell of an oyster and the 

 coat of armour of a lobster or a crab are familiar 

 examples of such outside skeletons. The Verte- 

 brates, on the other hand, are provided with an 

 inside skeleton deeply embedded in the body. In 

 some primitive Vertebrates in certain fishes, for 

 instance the inside skeleton is constructed of 

 cartilage, and never advances beyond the carti- 

 laginous stage. The presumption that the deep 

 body-cells of the Vertebrates have learnt, in some 

 way or another, to manufacture a cartilaginous 

 inside skeleton, and farther than that they cannot 

 go, is justifiable. When an inside skeleton of 

 greater rigidity than can be afforded by cartilage 

 becomes a functional necessity to a Vertebrate 

 it is possible that the surface-cells, who learnt the 

 business of manufacturing bone or bone-like sub- 

 stance in the Invertebrate ancestor untold ages ago, 

 migrate from the surface and take their share in 

 providing the Vertebrate with an inside skeleton. 

 This is, of course, pure hypothesis, but as such it 

 offers an explanation of the battle of the tissues 

 ultimately resulting in the replacement of cartilage 

 by bone, and proves, at any rate, that the problem 

 of the origin of the bone-building cells is one of 

 considerable interest. 



When the actual process of laying down 

 bone first begins, the bone-building cells seem 

 to be marshalled and ordered in a very definite 



p 225 



