Reproduction 



Osteoblasts Calcifying connective-tissue bundle 



285 



Fig. 235. Development of dermal (secondary) bone from mesenchyme. From 

 a section of the mandible of a human embryo of four months. (X 210.) (Modified 

 from Bremer: "Text-Book of Histology," Philadelphia, The Blakiston Company.) 



Iii the development of one of the long bones of an appendage, 

 perichondral osteoblasts deposit a shell of bone (perichondral bone) 

 on the external surface of the cartilage. Meanwhile, ingrowing exten- 

 sions of the perichondrium provide osteoblasts which build up endo- 

 chondral bone on the calcified remnants of the cartilage. 



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. Mesenchyme cells, becoming 

 osteoblasts, build up bone lamellas directly on the surfaces of strands 

 of calcified connective tissue (Fig. 235). Most of the bones which de- 

 velop in this manner are derived from the embryonic mesenchyme of 

 that same general superficial layer which otherwise gives rise to the 

 dermis of the skin. They are accordingly called dermal bones. Cer- 

 tain deep-lying bones (e.g., the clavicle of the shoulder girdle) which 

 develop directly from mesenchyme are called membrane-bones. 

 They presumably had phylogenetic origin from dermal bones. Bone 

 resulting from replacement of cartilage is called cartilage-bone. 



Mesenchyme is the source of nearly all nonstriated or "smooth" 

 muscle, whether in the walls of viscera or in the body-wall. Most 

 visceral organs are hollow. In their early embryonic stages, their pri- 

 mary and essential walls are either endoderm, as in the case of the 

 digestive tube, lung, or urinary bladder, or mesoderm, as in the urino- 

 genital ducts. The outer surfaces of these primary walls are always 

 adjacent to regions occupied by mesenchyme. The nonstriated muscle- 

 fibers of these organs are differentiated from cells of the adjacent 

 mesenchyme. Nonstriated muscle-fibers occur in the walls of larger 

 blood-vessels and of some integumentary glands, where they serve to 



