SKELETON. 



39 



Cartilages and bones are covered on their outer surfaces by an 

 envelope of connective tissue, called respectively perichondrium or 

 periosteum. These membranes form the means by which muscles 

 are attached to the bones and by which blood-vessels obtain entrance to 

 them. The periosteum is also a seat of bone formation. 



DERMAL SKELETON. 



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

 tion of cells at definite points in the corium. These cells become 

 specialized (scleroblasts, odontoblasts or osteoblasts) for the 

 deposition 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. 



FIG. 31. Cross-sections of developing scale of Acanthias. c, stratum corneum; d, dentine 

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



It is generally thought that the primitive dermal skeleton resembled 

 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. The surface cells of the papilla and the region 

 around it becomes converted into osteoblasts which secrete calcic 



