74 



GENERAL ZOOLOGY 



nective tissue without passing through a cartilaginous stage. 

 The greater part of the skeleton is laid down in cartilage, which, 

 except in those forms in which the skeleton is permanently 

 cartilaginous, is replaced by bone tissue formed about the 

 cartilage as the latter is resorbed. In the frog some of the primi- 

 tive cartilaginous skeleton survives in the adult, though the 

 greater part of it is converted into bone. Membrane bones, such 

 as those of the face, the flat bones of the skull, parts of the jaw, 

 etc., may be formed regardless of whether the original carti- 



Fig. 46. — Cross section of compact bone from the shaft of the humerus show- 

 ing the ground substance in white. The large black areas are occupied by blood 

 vessels, the bone cells lying in the smaller spaces arranged in concentric circles. 

 X 150. (From Schafer, Textbook of Microscopic Anatomy, Longmans, Green & 

 Co., after Sharpy. By permission.) 



laginous skeleton becomes ossified or not. The calcified cartilage 

 of sharks is almost as hard as bone. 



The nature of the process of histogenesis is the same for both 

 cartilage and membrane bone. The preliminary formation of 

 cartilage in the formation of cartilage bone probably signifies the 

 existence of a primitive ancestral cartilaginous skeleton in which 

 the homologues of the membrane bones were not represented. 

 Therefore, membrane bone would seem to be a later addition to 

 the skeleton. 



A cross section of a decalcified human long bone (Fig. 46) 

 shows large black spaces, Haversian canals, surrounded by 

 concentric circles of smaller spaces, lacunae, with fine radiating 

 lines connecting the circles. The Haversian canals contain blood 



