194 EMBRYOLOGY 



shall first follow the fate of a somite. Shortly after its formation some of the 

 cells of the somite begin to migrate in the direction of the notochord and 

 neural tube. These cells form a mass of mesenchyme which is called 

 sclerotome. The sclerotome finally completely surrounds the notochord and 

 neural tube and differentiates into the vertebrae, as shown in Figure 116. 



Another portion of the somite forms a discrete layer of cells and is termed 

 the dermatome. It gives rise to the mesodermal portion of the skin, the 

 corium. The cells of the dermatome form migratory mesenchyme cells which 

 cover the inner surface of the epidermis. The core of the feather and the 

 connective tissue layer of the skin develop from this mesenchyme. 



The remainder of the somite forms the myotome, a layer of cells which 

 develops into muscle. Not all the muscle of the body comes from the 

 myotome, since, as we have already seen, the muscle of the heart differentiates 

 from the cells of the splanchnic mesoderm. The limb muscles develop from 

 mesenchyme which has its origin in the somatic layer of the mesoderm. 

 Smooth muscle of the digestive tract comes from splanchnic mesoderm, and 

 finally, in the head region, the musculature is formed by the head mesen- 

 chyme. Thus the myotome of the somite differentiates mainly into the 

 voluntary muscles of the back. 



Differentiation of the nephrotome 



The second of the primary divisions of the body mesoderm is the 

 nephrotome, or the intermediate mesoderm. This tissue undergoes a compli- 

 cated series of changes leading, first, to a pronephros, which is nonfunctional; 

 next, a mesonephros, which functions for several days; and finally, a meta- 

 nephros, which persists as the permanent kidney of the adult. The resem- 

 blance of the pronephros of the very early chick embryo to the pronephric 

 kidney in lower fishes is very striking, even though the chick pronephros is 

 solid and thus nonfunctional. 



As Figure 117 shows, the pronephric tubules develop from the nephro- 

 tome between the 5th and the 16th somites, and a solid pronephric duct 

 forms in this region. The pronephric duct grows posteriorly and becomes the 

 hollow mesonephric duct leading into the cloaca. The pronephros is a transi- 

 tory structure, and the anterior end begins to break up soon after its forma- 

 tion. It is very difficult to see why the pronephros should form at all. Perhaps 

 it is something like the notochord, which simply serves as a structure around 



