CARTILAGE AND BONE 65 



inner surface of the periosteum (perichondrium of earlier stages). 

 Intramembranous bones are formed, but usually to a much smaller 

 extent, in the region of the skull, the pectoral girdle, and exo- 

 skeleton generally. 



How a rigid structure like bone can grow while in continuous 

 functional activity, and still retain its shape while increasing in 

 size, are questions which soon suggested themselves. Certain 

 authors, like Todd and Bowman, Strelzoff and Kastschenko, held 

 that bones enlarge by interstitial growth and expansion ; others, 

 like Hunter, Kolliker, and Stieda, supported the opposite, and now 

 generally accepted, view that bone once formed does not expand, 

 but grows by the deposition of new layers on its surface. While 

 the osteoblasts are continually adding new material in one place, 

 other cells, termed osteoclasts, may be destroying the older 

 portions in another. Thus a bone may alter in shape, or may 

 increase in size, retaining its form, as the needs of the animal 

 demand. In this way the greatest mechanical efficiency is secured, 

 with the least weight and expenditure of material, all superfluous 

 bony substance being removed in the formation of medullary 

 cavities. 



There remain, perhaps, a few cases in which bone is formed by 

 the actual conversion of cartilage with its cells. It has been 

 described by Schmid-Monnard [388a] in Teleostean fish, by 

 Kastschenko in Amphidia, and by Gegenbaur [153] in Mammalia. 

 But even here appearances are probably deceptive. Stephan [424] 

 has explained how apparent conversion may be due to the gradual 

 change of ' perichondrium ' into ' periosteum ' ; so that if this 

 layer ceases to produce cartilage and takes to producing bone, 

 sections show a gradual transition from one to the other. Thus, 

 it would be the activity of the growing tissue which changes, 

 not the already formed skeleton which undergoes conversion. 



A comparative study of the general development of the skeleton 

 was begun by Duges in Amphibia [123]; by von Baer, Eathke, 

 Reichert, Jacobson, Kolliker, Parker [323, etc.] in various groups 

 of vertebrates. They concluded that the history of the development 

 of a bone affords important evidence concerning its homology. The 

 'cartilage-bones,' connected with the endoskeleton, were called 

 ' primary ' ; the ' membrane-bones,' which appeared to have been 

 added from without, were called ' secondary.' It was argued that 

 homologous bones must develop in a similar way ; in other words, 

 it was thought that a ' primary ' bone could not be homologous 

 with a ' secondary ' bone. This morphological distinction, between 

 bone developed inside the perichondrium and bone developed more 

 superficially, was found to be so strongly supported by the facts, 

 that it became almost a dogma that bones of unlike development 

 could not be homologous. 



