THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 169 



cells become more widely separated from one another, at the same time assum- 

 ing oval or spindle shapes and then irregular branching forms (Fig. 144). 

 The rest of the mesoderm, except the mesothelium, also undergoes a similar 

 transformation so that structurally its cells are indistinguishable from those 

 derived from the sclerotomes and cutis plates. 



Thus the tissue from which the connective tissues in general are derived is 

 composed at one stage of irregular branching cells, with a relatively large 

 amount of a homogeneous substance filling the interstices among the cells. 

 The question of the relation of these cells to one another has not been settled 



m 





FIG. 144. Mesenchymal tissue from somatopleure of a 5 mm. human embryo. 

 Mesothelium is shown along lower border of figure. 



By some it is held that they are simply individual units, structurally independ- 

 ent of one another. By others it is maintained that the branches of each 

 cell anastomose with branches (5f neighboring cells, to form a syncytium, and that 

 the syncytial character is retained in the connective tissue derivatives (Mall). 

 That intercellular substance is derived originally from the cell can scarcely 

 be denied. All the cells of the organism are derived from the fertilized ovum. 

 As soon as two or more cells are formed by segmentation of the ovum, they are 

 either simply in apposition or else they are united by something in the nature of 

 a "cement" substance which must have been derived from the cells themselves. 

 In the connective tissues this intercellular ground substance is a prominent 



