54 ZOOLOGY. 



point. The bone-corpuscles or cells (also called osteoblasts) 

 are arranged in concentric cylinders around each Haversian 

 canal. Each is a much-branched cell : in sections of dead, 

 dried bones, the places occupied by the bone-corpuscles 

 are seen as little cavities, to which the name of lacunae 

 was given long before their meaning was understood. The 

 lacunae are connected together by very fine canals, the 

 canaliculi, by means of which the plasma exuded from the 

 Haversian vessels can reach all the bone-corpuscles. 



Forming a close sheath round the bone is a membrane, 

 the periosteum, the outer layers of which are of the nature 

 of ordinary connective tissue, while the inner ones consist of 

 closely packed cells. 



The rigidity of the matrix prevents bone from growing, 

 as cartilage does, by cell-division and intercalation of new 

 matrix-material. Growth of a bone is partly effected on 

 the outside by the activity of the inner layer of periosteum 

 just mentioned. But there are more complicated arrange- 

 ments at work, which will be better understood after we 

 have described the development of bone. 



10. Membrane-bone. The simplest mode of origin of 

 bone is found in the case of most of the flat bones of the 

 skull. In the embryo rabbit there exists in the region 

 where these bones will appear a kind of connective tissue 

 with but little matrix. The corpuscles of this tissue after a 

 time begin to secrete calcium phosphate as well as collagen, 

 and bone is thus produced. Such a bone is called a 

 membrane-bone. 



11. Cartilage-bone. In the case of the majority of the 

 rabbit's bones many of those of the skull, all the vertebrae, 

 ribs, limbs, and limb-girdler a much more complicated 

 process occurs. These bones are called, by distinction from 

 membrane-bones, cartilage-bones. Each one of these bones 

 has preceding it in the same position in the embryo a mass 

 of cartilage of the same shape, surrounded by a " perichon- 

 drium" resembling the tissue described in the previous 

 paragraph. After a time both the cartilage-corpuscles and 

 the corpuscles of the perichondrium begin to deposit calciuin 



