SKELETON 



43 



periosteum. These membranes form the means by which muscles 

 are attached to the skeleton, and by which blood-vessels obtain 

 entrance to the hard parts. The periosteum is also a seat of bone 

 formation. ^'^-^-^ " 



DERMAL SKELETON 



When present, the dermal skeleton arises by a marked prolifera- 

 •ion of cells at definite points in the corium. These cells become 

 specialized (scleroblasts, odontoblasts or osteoblasts) for the deposi- 

 tion of salts of lime plus a varying amount of organic matter (ossein). 

 Upon limy plates formed in this way other parts, also calcareous, 

 may be laid down by the basal surface of the epidermis, so that 

 the whole dermal element may be in part mesenchymatous, in part 

 ectodermal in origin. 



I IG. 34. — Cross-sections of developing scale of AcatUhias. c, stratum corneum; d, 

 dentine of scale; ee, enamel organ; tn, stratum Malpighii; p, pulp. 



It is generally thought that the primitive dermal skeleton re- 

 sembled that of existing sharks, and that from the hypertrophy or 

 fusion of such scales the so-called membrane bones have arisen. 

 Then the scales of other vertebrates are to be traced back to an 

 elasmobranch ancestry, while teeth are thought to be modified scales. 

 Hence the structure and development of the elasmobranch scale 

 should be understood. 



At regular intervals in the skin of a shark there is a multiplication 

 of cells of the corium, each aggregation forming a small papilla which 

 projects above the surrounding corium, carrying with it the basal 

 layer of the epidermis (fig. 34). The surface cells of the papilla 

 and the region around it become converted into osteoblasts which 

 secrete calcic salts on their outer ends, thus forming a small plate of 

 dentine (p. 28) with a central spine into which the papilla extends. 

 The overlying epidermal cells form an enamel organ, the lower 



