54 



HISTOLOGY. 



the skeleton consists of hyaline cartilages which correspond with the bones 

 of the adult. Around the cartilages, or in some places quite apart from 

 them, the bone is formed in the following manner: 



Calcifying connective 

 tissue bundles. 



Bone cells. 





> c. 

 FIG. 58. FROM A SECTION OF THE MANDIBLE OF A HUMAN FETUS FOUR MONTHS OLD. X 240. 



In the embryonic connective tissue certain homogeneous strands 

 become apparent, staining deeply with eosin, Fig. 58. These represent 

 the matrix or ground substance of bone, and are considered either trans- 

 formations of the exoplasm of the neighboring cells, or as secretions of those 



cells, or as modifications 

 of connective tissue fibrils. 

 They blend with the con- 

 nective tissue as shown in 

 the lower part of the figure. 

 As these strands become 

 distinct, they are seen to be 

 covered with peculiar cells of 

 mesenchymal origin which 

 tend to form a distinct 

 layer. Since they produce 

 bone they are called osteo- 

 blasts. (Blast is a designa- 





Bone matrix. 



Osteoblasts. 



Bone cell. 



:."^ilS> 



"'-:-^ 



T 



FIG. 59. PART OF A CROSS SECTION OP_THE SHAFT OF THE 

 HUMERUS OF 



HUMAN EMBRYO FOUR MONTHS OLD. 



tion for a formative cell, and 

 is used in many combina- 

 tions with a prefix denoting 

 the structure which it pro- 

 duces.) Osteoblasts are 



shown in Figs. 58 and 59. They are cells with rounded nuclei and abun- 

 dant protoplasm, varying in shape from flat to columnar, often being 



X 560. 



