THE CONNECTIVE TISSUES. 



117 



of bone is deposited by the perichondrium (in a manner to be 

 described under the head of intramembranous bone-development) 

 and the perichondrium becomes the periosteum. This in the mean- 

 time has differentiated ino two layers an outer, consisting largely 

 of fibrous tissue with few cellular elements, and an inner, the 

 osteogenetic layer, vascular and rich in cellular elements and con- 

 taining few fibrous-tissue. fibers. 



Ossification in the cartilage begins after the above-described 



Vesicular cartilage- 

 cells. 



-Primary periosteal 

 bone lamella. 



Periosteal bud. 



Fig. 85. Longitudinal section through a long bone (phalanx) of a lizard embryo. 

 The primary bone lamella originating from the periosteum is broken through by the peri- 

 osteal bud. Connected with the bud is a periosteal blood-vessel containing red blood- 

 corpuscles. 



structural changes have taken place at the center of ossifica- 

 tion. Its commencement is marked by a growing into the cartilage 

 of one or several buds or tufts of tissue derived principally from 

 the osteogenetic layer of the periosteum. As the periosteal buds 

 grow into the cartilage, some of the septa of matrix separating the 

 altered cartilage-cells disappear, and the cells become free and 

 probably degenerate. In this way the cartilage at the center of ossi- 



