Embryology 289 



Mesenchyme is a particular type of embryonic cell which is pro- 

 duced principally by the mesoderm, but may be formed by any of the 

 germ layers. Mesenchymal cells are branched and more or less united. 

 When they break away from the germ layers, they become irregular 

 in shape and by ameboid movements are able to move to definite regions 

 and reorganize. Thus an organ derived from mesenchyme may actually 

 come from more than one layer. The mesenchyme gives rise to all 

 the connective tissues including the skeletal system, all the involuntary 

 muscles, blood, and lymph vessels, and the voluntary muscles of the 

 appendages. The involuntary muscles of the digestive tract are derived 

 from mesenchyme cells produced from the splanchnic mesoderm. Blood 

 cells and parts of the dermis likewise have their origin from mesenchyme. 



The Embryonic Germ Layers and Their Derivatives 



The germinal epithelium cannot be listed as being derived from any one 

 germ layer inasmuch as the primordial germ cells are set aside before the germ 

 layers are defined. 



