EMBRYO OF SEVENTEEN SEGMENTS 55 



into the pericardial cavity about the heart, the pleural cavity of the thorax, 

 and the peritoneal cavity of the abdominal region. In the chick stages 

 already studied, the embryo was flattened on the surface of the yolk and 

 and the somatopleure and splanchnopleure did not meet ventrally. If this 

 union occurred they would conform to the structural relations shown in 

 Fig. 54, which is essentially the ground plan of the vertebrate body. 



Mesenchyme. In the sections through the head of this embryo, and 

 through that of the preceding stage, but four primitive segments were found. 

 The greater part of the mesoderm in the head appears in the form of an 

 undifferentiated network of cells which fill in the spaces between the defi- 

 nite layers (epithelia). This tissue is mesenchyme (Fig. 55). The meso- 

 derm may be largely converted into mesenchyme, as in the head, or any of 

 the mesodermal layers may contribute to its formation. Thus, it may be 



Ectoderm 



Alesenchyme 



FIG. 55. Mesenchyme from the head of a thirty-eight-hour chick embryo. X 495. 



derived from the primitive segments and from the somatic and splanchnic 

 mesoderm. The cells of the mesenchyme form a syncytium, or network, 

 and are at first packed closely together. Later, they may form a more 

 open network with cytoplasmic processes extending from cell to cell (Fig. 

 55). The mesenchyme is an important tissue of the embryo; from it are 

 differentiated the blood and lymphatic systems, together with most of the 

 smooth muscle, connective tissue, and- skeletal tissue of the body. 



The body of the embryo is now composed: (i) of cells arranged in 

 layers epithelia, and (2) of diffuse mesenchyme. The term 'epithelium' 

 is used in a general sense. Those epithelial layers lining the body cavities 

 are termed mesothelia, while those lining the blood vessels and lymphatics 

 are called endothelia. 



Derivatives of the Germ Layers. The tissues of the adult are de- 

 rived from the epithelia and mesenchyme of the three germ layers as 

 follows : 



