526 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS 



Stages. At the time of tail-rudiment formation, tliis mesoderm forms two bi- 

 lateral masses of cells located within the "tail bud" or "end bud." These cellular 

 masses proliferate extensively as the tail bud grows caudally and give origin 

 to the mesoderm of the tail. Similarly, in other vertebrates, the mesoderm of 

 the future tail is present as mesenchyme in the terminal portion of the tail 

 bud. These mesenchymal cells proliferate, as the tail grows caudalward, and 

 leave behind the mesoderm, which gradually condenses into the epithelial 

 masses or segments (myotomes) along either side of the notochord and 

 neural tube. 



4. Contributions of the Trunk Mesoderm to the 

 Developing Body 



The mesoderm of the trunk area contributes greatly to the development of 

 the many body organs and systems in the trunk region. Details of this con- 

 tribution will be described in the chapters which follow, but, at this point, it 

 is well to survey the initial activities of the mesodermal tubes of the trunk 

 area in producing the vertebrate body. 



a. Early Differentiation of the Somites or Epimere 



The somites (figs. 217, 237, 252) contribute much to the developing struc- 

 ture of the vertebrate body. This fact is indicated by their early growth and 

 differentiation. For example, the ventro-mesial wall of the fully developed 

 somite gradually separates from the rest of the somite and forms a mass of 

 mesenchymal cells which migrates mesad around the notochord and also 

 dorsad around the neural tube (fig. 252A-C). The mesenchyme which thus 

 arises from the somite is known as the sclerotome. In the somite of the higher 

 vertebrates just previous to the origin of the sclerotome, a small epithelial 

 core of cells becomes evident in the myocoel; this core contributes to the 

 sclerotomic material (fig. 252B). As a result of the segregation of the sclero- 

 tomic tissue and its migration mesad to occupy the areas around the notochord 

 and nerve cord, the latter structures become enmeshed by a primitive skele- 

 togenous mesenchyme. The notochord and sclerotomic mesenchyme are the 

 foundation for the future axial skeleton of the adult, including the vertebral 

 elements and the caudal part of the cranium as described in Chapter 15. 



After the departure of sclerotomic material, myotomic and dermatomic 

 portions of the somite soon rearrange themselves into a hollow structure (fig. 

 252C, D), in which the myotome forms the inner wall and the dermatome 

 the outer aspect. This composite structure is the dermomyotome, and the 

 cavity within, the secondary myocoel. In many vertebrates (fishes, amphibia, 

 reptiles, and birds), the dermatome gives origin to cells which migrate into 

 the region of the developing dermis (Chap. 12) and contributes to the forma- 

 tion of this layer of the skin. 



