BONE 87 



The processes of bone formation and resorption just described take 

 place both in membrane and in cartilage bones. As the membrane bones 

 enlarge, the central portion, through resorption, becomes loose spongy bone 

 (substantia spongiosa), which is enclosed on all sides by an outer layer of 

 compact bone (substantia compacta). In the flat bones of the skull the 

 compact substance forms the outer and inner " tables, " which have the 

 spongy "diploe" between them. The cartilage bones likewise consist 

 of spongy and compact portions. 



Hyaline 

 cartilage. 



Primary 

 marrow 

 space. 



Perichondrial 

 bone. 



PlG. 74- A DORSO-PALMAR LONGITUDINAL SECTION OF A PHALANX OF THE LITTLE FlNGER, FROM A HUMAN 



EMBRYO OF THE SIXTH MONTH. X 60. 



Replacement of the skeletal cartilages. The changes within the skeletal 

 cartilages during the formation of bone may be studied advantageously 

 in longitudinal sections of any developing "long bone," or in transverse 

 sections of the vertebrae from pig embryos measuring about 10 cm. The 

 vertebiae exhibit several processes which will be cut lengthwise in trans- 

 verse sections. Fig. 74 represents a longitudinal section of a phalanx 



