DEVELOPMENT OF SKELETAL TISSUES 663 



(lamellae) with their associated lacunae, osteocytes, and canaliculi (fig. 

 314B-D). Blood vessels from the marrow cavity within the bone or from 

 the surface of the bone via Volkmann's canals (fig. 314D) pass into the 

 Haversian canals, thus supplying nourishment and other life-maintaining 

 features to the canaliculi and through the latter to the osteocytes. Compact 

 bone thus restricts the marrow cavity to a central area, and the Haversian 

 and Volkmann canals convey the blood supply into the compact bony sub- 

 stance which surrounds the central marrow cavity. In general, the Haversian 

 systems are formed parallel with the long axis of the bone. Circumferential 

 lamellae surround the external surface of the bone around the Haversian 

 systems. Inner circumferential lamellae also are present lining the marrow 

 cavities of long bones. 



C. Development of Skeletal Tissues 



1. Formation of the Connective Tissue Proper 



a. Formation of Fibrous Connective Tissues 



In the early embryo, following the ghost-skeleton stage, two types of con- 

 nective tissues are found: 



( 1 ) Mucoid or loose connective tissue is located in Wharton's jelly in 

 the umbilical cord of mammals and in other parts of the embryo. This 

 embryonic type of connective tissue is characterized by the presence 

 of large mesenchymal cells whose processes contact the processes of 

 other surrounding mesenchymal cells (fig. 312A). Within the mesh- 

 work formed by these cells and their processes, mucus or a jelly-like 

 substance is present. Very delicate fibrils may lie within this jelly. 



(2) A second type of early embryonic connective tissue is reticular tissue. 

 It contains stellate mesenchymal cells whose processes contact each 

 other (fig. 312B). Very delicate bundles of fibrils may be present 

 which are closely associated with the cells. 



The foregoing, connective-tissue conditions of the early embryo eventually 

 are replaced by the mature forms of connective tissue. In this process the 

 reticular type of connective tissue appears to form an initial or primary stage 

 of connective-tissue development. For example, in the development of white 

 fibrous tissue, a delicate network of fine fibrils appears within the ectoplasmic 

 ground substance between the primitive mesenchymal cells, thus forming a 

 kind of reticular tissue (fig. 312A, B). With the appearance of fibrils be- 

 tween the mesenchymal cells, the latter may be regarded as fibroblasts. Fol- 

 lowing this reticular stage, the ectoplasmic ground substance becomes more 

 fibrillated and parallel bundles of white fibers arise, probably by the direct 

 chemical transformation of the earlier fibrils into white or collagenous fibers 

 (fig. 312C). (See Bardeen, '10, p. 300.) It is probable that the elastic con- 



