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 cellsbecome 
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. 
Fic. 31.—Cross-sections of developing scale of Acanthias. c, stratum corneum; d, dentine 
of scale; ce, enamel organ; m, stratum Malpighii; , 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 
