CH. XIV] ORGANS FROM THE MESODERM 147 



Over the anterior end of the embryo and around the pharynx 

 the mesoderm forms a thin layer of cells, loosely held together 

 (Fig. 26, B). The mesoderm over the dorsal surface of the 

 pharynx and beneath the brain plate is represented by only a 

 single layer of somewhat scattered cells. Around the blasto- 

 pore there is a thick layer of mesodermal cells which is thickest 

 on the dorsal surface. In general, in the posterior region of the 

 body the mesoderm is thicker than in the middle and anterior 

 regions. 



THE MESODERMIC SOMITES 



In the following stages of development of the embryo the 

 dorsal ectodermal plate is lifted up and rolled in to form the 

 central nervous system (Fig. 42). The mesoderm lying on 



Nc 



FIG. 42. Cross-section through middle of embryo. M. Medullary plate. N. Noto- 

 chord. Xc. Xeural crest. PS. Primitive segment-plate. SO, SP. Somatic and 

 splanchnic niesoderm. 



each side of the notochord changes shape somewhat during this 

 time. It forms on each side a thick, nearly solid mass of cells, 

 the plate of the primitive segments or segmental plate (Fig. 

 42). The outermost cells of this mass, i.e. those lying nearest 

 to the dorsal surface, now show a tendency to arrange them- 

 selves into an epithelial layer. This layer is at first continu- 

 ous at the sides with the outer or somatic layer of cells of 

 the lateral mesodermal sheets. The two layers of cells of the 

 lateral mesodermal sheets (Fig. 42, SO and SP), the somatic 

 and splanchnic layers, often show a tendency to separate and 

 leave a cavity between them. This cavity filled with fluid 



