THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 167 



segments and ccelom, the mesoderm simply fills in the space between the 

 ectoderm and entoderm (Fig. 140). Portions of the mesoderm in all these 

 regions are destined to give rise to connective tissues. Each primitive segment 

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

 myotome (Fig. 142). Of these, only the sclerotome and cutis plate are directly 

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

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



mitive segment 



Intermediate 

 cell mass 



Intermediate .. 

 cell mass 



Visceral 





M sothelium * "J 



^} Lateral 



. i-tfT" body wall 



■ Umbilical vein 



Fig, 141. — 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 (except the mesothelium lining the coelom) 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 primitive 

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

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



