200 EMBRYOLOGY 



The lateral mesoderm 



We have yet to consider the third component of the primary mesoderm. 

 The lateral mesoderm forms two distinct layers of cells: an outer layer of 

 somatic mesoderm and an inner layer of splanchnic mesoderm. The space 

 between these layers is the body cavity, or coelom. These two layers differ 

 greatly in the structures which they form during development. In general, 

 the somatic layer forms muscle, such as that of the limbs, and some mesen- 

 chyme of the inner layer of the skin. The splanchnic layer, on the other 

 hand, forms smooth muscle and connective tissue of the viscera. The 

 mesenteries supporting the viscera also arises from splanchnic mesoderm 

 and serve as pathways for blood vessels and nerves. Both layers contribute 

 to the formation of the peritoneum. 



These two layers are stimulated to differentiate by contact with the 

 endoderm and the ectoderm. For example, if the endoderm of an amphibian 

 neurula is removed, the splanchnic layer of mesoderm develops just as the 

 somatic layer does and it forms limbs. Thus, the endoderm induces the 

 differentiation of the splanchnic mesoderm. As a matter of fact, the splanch- 

 nic and somatic layers may be interchanged in early development and a 

 normal embryo will still result. This experiment tells us that there is no 

 determination in these layers in early development and that their differentia- 

 tion depends on contacts with the surrounding tissues. 



Once determined, the derivatives of the splanchnic mesoderm are many. 

 In the chick embryo we have already seen that the blood islands, comprising 

 both blood and lining of blood vessels, originate in this layer. During the 

 formation of the heart the lateral mesoderm closes in on the blood vessels 

 and differentiates into the muscle of the heart. Much later in development 

 the spleen forms from the splanchnic mesoderm. 



As the differentiation of the digestive tract and its derivatives proceeds, 

 the splanchnic mesoderm becomes involved in the formation of the smooth 

 muscle and connective tissue of the digestive tube. The mesenteries which 

 support the viscera and which conduct nerves and blood vessels to the organs 

 also differentiate from this layer. 



As development continues, the splanchnic mesoderm becomes converted 

 into peritoneal epithelium and from this layer the germinal epithelium arises. 

 The germinal epithelium differentiates into the bulk of the ovary and also 



