REPRODUCTION 



105 



Since the perichondrium is produced by mesenchyme, the bone is indirectly 

 a derivative of mesenchyme. 



In the development of certain of the more superficial bones of the 

 cranium, the outer bones of the jaw skeleton and some parts of the shoulder 

 girdle, no cartilage is formed. At the site of the prospective bone, 

 mesenchyme produces a fibrous tissue whose fibers become calcified 

 (Fig. 78). Mesenchyme cells, becoming osteoblasts, congregate on the 

 surfaces of the strands or spicules of this calcified spongy matrix and, 

 using it as a foundation, build up successive delicate lamellae of the 

 calcareous material which constitutes bone. Occasionally an osteoblast 

 will become enclosed in a minute space or lacuna between bone lamellae 

 and persists there as a bone cell. 



Osteoblasts. Calcifying connective tissvie bundles. Bone matrix. 



Bone cells . 



Fig. 78. — Development of dermal (secondary) bone from mesenchyme. From a 

 section of the mandible of a human embryo of four months. X240. (From Bremer, 

 Text-book of Histology.) 



Most of the bones which develop in the manner last described are 

 derived from the embryonic mesenchyme of that same general superficial 

 layer which otherwise gives rise to the dermis (or corium) of the skin. 

 They are accordingly called dermal bones. In course of evolution they 

 apparently originate by expansion and fusion of the basal plates of 

 calcareous placoid scales of elasmobranchs. Certain bones — for example, 

 the clavicle of the shoulder skeleton — which develop without intervention 

 of cartilage occur in positions so deep that the bones are not obviously and 

 literally "dermal." They are called membrane bones, a term which is, 

 however, often expanded to include all bones which arise directly from 

 mesenchyme. Presumably deep "membrane" bones had their phyloge- 

 netic origin in dermal bones. Assuming this, all bones developed directly 

 from mesenchymatous fibrous tissue may be called dermal, as is commonly 

 done. It follows from all of this that no distinction can be maintained 



