124 OUTLINES OF CHORDATE DEVELOPMENT 



this splits into two more or less distinct sheets, the somatic 

 and splanchnic layers, in contact with ectoderm and endoderm 

 respectively. The space between these is the splanchnocu>l, 

 the rudiment of the body cavity or ccelom. In the stage we 

 are describing the coelom does not extend completely through 

 the lateral plate as it does finally. The ccelom does however 

 extend into the segmental plate, as the rudiment of themyocoels, 



nc 



so 



ec 



FIG. 39. Part of a section through the anterior body region of an embryo of 

 R. sylvatica, just beginning to elongate, illustrating the differentiation of the 

 mesoderm. After Field, c, Coslom; ec, ectoderm; en, endoderm; g, gut cavity; 

 mp, medullary plate; my, myotome; n, notochord; nc, rudiment of neural crest; 

 so, somatic layer of mesoderm; sp, splanchnic layer of mesoderm. 



appearing toward the surface of this thicker mass. As the 

 neural folds approach, the cells of this axial mesoderm are 

 rearranged in such a way that the longitudinal bands are cut 

 transversely into segments or somites. This process begins 

 just back of the pharynx and extends rapidly toward the 

 posterior ends of the bands (Figs. 44, E; 53). It is important 

 that the mesoblast of the head region is not clearly divided into 

 segments. By the time the neural tube is completed three or 

 four pairs of somites have been formed. At first these are 



