THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 



131 



somites and ccelom, the mesoderm simply fills in the space between the ecto- 

 derm and entoderm (Fig. 102). Portions of the mesoderm in all these re- 

 gions are destined to give rise to connective tissues. Each mesodermic somite! 

 soon becomes differentiated into three parts the sclerotome, cutis plate and ( 

 myotome (Fig. 104). Of these, only the sclero tome and cutis plate are directly 

 concerned in the formation of connective tissues, the myotomes giving rise to 

 striated voluntary muscle. The sclerqtomes are destined to give rise to the 



Neural tube 



tf \ Primitive segment 



Intermediate 

 cell mass 



Visceral mesoderm ft 



Mesothelium 



f'^Xx^ 



Lateral 

 body wall 



Umbilical vein 



FIG. 103. Transverse section of human embryo with 13 primitive segments; section taken 

 through the 6th segment. Kollmann. 



vertebrae and other forms of connective tissue in their neighborhood, the cutis 

 plates to a part, at least, of the corium of the skin. The parietal and visceral 

 layers of the mesoderm (excepFthe mesothelium lining the ccelom) and the 

 mesoderm of the head region are destined to give rise to the various types of 

 connective tissue forming parts of the other organs of the body. 



HISTOGENESIS. 



The sclerotomes and cutis plates at first constitute parts of the mesoorermic 

 somites, and are composed of epithelial-like cells with little intercellular sub- 

 stance. The intercellular substance gradually increases in amount so that the 



