REPRODUCTION 



75 



cartilaginous in elasmobranchs. Cartilage is a direct product of mesen- 

 chyme. Cells of the mesenchyme become cartilage cells (Fig. 68) and 

 deposit the ground substance or matrix of the cartilage. In the great 

 majority of vertebrates the primary cartilaginous skeletal structures are, 



Mes. PrcCart. Cart. 



A'- 



m. 



0': 





'"^^^^^j|^ 



Fig. 68. — Diagrams illustrating formation of cartilage by mesenchyme. A, in fishes, 

 according to Studnicka; B, in mammals, according to Mall. Cart., cartilage; Mes., 

 mesenchyme; Pre. Cart., precartilage. (From Bremer, "Text-book of Histology.") 



in later development, more or less completely replaced by bone. The 

 process of replacement (Fig. 178) involves the destruction of the greater 

 part of the cartilage. The remnants of the cartilage are in form of a 

 spongy meshwork whose strands become calcified and serve as a frame- 

 work upon which bone-producing cells, osteoblasts, build up bone. 



Osteoblasts. 



Calcifying connective-tissue bundles. Bone matrix. 



Bone cells. 



Fig. 69. — 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.") 



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 directly on the surfaces of strands of calcified connective 



