172 



TEXT-BOOK OF EMBRYOLOGY. 



bone Howslip's lacuna (Fig. 152). They apparently possess the power of 

 dissolving bone tissue. While the destruction of bone by the osteoclasts 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 continues to enlarge 



Cartilage 



Osteogenetic tissue 



Intracartilaginous 

 bone 



Subperiosteal 

 bone 



Blood vessels 



Periosteum 

 (perichondrium) 



/ Ossification center 



Calcification zone 



FIG. 153. Longitudinal section of one of the metatarsal bones of a sheep embryo. 

 (Intracartilaginous ossification.) 



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 



