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TEXT-BOOK OF EMBRYOLOGY. 



bone Howslip's lacuna (Fig. 114). Whether they are the specific agents in 

 dissolution of bone has been questioned (Arey). While the destruction of 

 bone is going on on the inner surface, new bone is being formed on the outer 

 surface, especially under the periosteum where the osteoblasts are most 

 numerous. Thus the layer of bone gradually comes to lie farther and 

 farther out and the cranial cavity is enlarged. So long as the cranial cavity 



Cartilage 



Osteogenetic tissue 



Intracartilaginous 

 bone 



Subperiosteal 

 bone 



Blood vessels 



Periosteum 

 (perichondrium) 



* Ossification center 



Calcification zone 



FIG. 115. Longitudinal section of one of the metatarsal bones of a sheep embryo. 

 (Intracartilaginous ossification.) 



continues to enlarge the new bone is of the spongy variety, but toward the end 

 of development the trabeculae become thicker and finally come together to 

 form the compact bone characteristic of the roof of the skull. The fact that 

 the new bone laid down during the enlargement of the cranial cavity is laid 

 down under the periosteum has led to the term subperiosteal ossification. 

 The process is essentially the same as in the original intramembranous 

 ossification. 



INTRACARTILAGINOUS OSSIFICATION. 



^ In this type of ossification hyalin cartilage is first formed in a shape which 

 ^ corresponds very closely to the shape of the future bone. For example, the 

 ! femur is first represented by a piece of hyalin cartilage which develops from 



