THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 165 



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

 assuming 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 mesoderm at this stage is composed of irregular, branching 

 cells, with a relatively large amount of homogeneous intercellular substance 

 filling the interstices. The branches, or protoplasmic processes, of each 

 cell anastomose freely with those of other cells in the immediate vicinity. 



I 





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

 Mesothelium is shown along lower border of figure. 



In this manner a syncytium is formed to which the term mesenchyme is ap- 

 plied (Fig. 144). The mesenchyme itself lacks specialization, being what 

 is known as an indifferent tissue, but it constitutes the structural basis upon 

 which all the connective tissues of the adult body are built ; all the forms of 

 connective tissue (except neuroglia) develop from it. 



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 them- 

 selves. In the mesenchymal tissue this intercellular ground substance is a 

 prominent feature, and probably represents in part nutritive materials and 

 in part the products of cell activity. 



