EMBRYO OF SEVENTEEN SEGMENTS 



53 



(dorsal) and the splanchnic (ventral) mesoderm (Fig. 34). These layers pei^ 

 sist in the adult, the somatic mesode rm giving rise to the p ericardium of the 

 heartj to the parietal pleura^rfjhe thorax and to the perit oneum of the abdomen, 



Notochord 



Nephrotome 



Arckenteron 



Splanchnic 

 mesoderm 



Entoderm' 



Neural tube 



Mesodermal segment 



Ectoderm 



Somatic 

 mesoderm 



Yolk 



5c.v>.J.^|> 



while the splanchnic layer forms the ep icardium a nd myocardium of the , 

 the viscexaljileura of the lungs, and the mesenteries and mesnHprmal l^ij^pr p f the 

 .gut^ The somatic mesoder m and the ectoderm, wit h the tissue developed be- 

 tween them, constitute the body wall, which is termed the somatopleure. In 

 the same way the splanchnic mesoderm and the entoderm, with the mesenchymal 

 tissue between them, constitute the wall of the gut, or the s planchnopleure. 



Ccelom. — The cavity between the s omatopleu reand splanchnopleure is the 

 ccelom (body cavity). With the splitting of the mesoderm, isolated cavities are 

 produced. These unite on 

 each side and eventually 

 form one cavity — the cce- 

 lom. With the extension 

 of the mesoderm, the cce- 

 lom surrounds the heart 

 and gut ventrally (Fig. 54) . 

 Later, it is subdivided into 

 the pericardial cavity about 

 the heart, the pleural cavity 

 of the thorax, and the 

 peritoneal cavity of the ab- 

 dominal region. In the 

 stages already studied, the 

 embryo was flattened on 



the surface of the yolk 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 net- 

 work of cells which fill in the spaces between the definite layers (epithelia). This^ 

 tissue is mesenchyme (Fig. 55). The mesoderm 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 derived from the primitive segments and 



eu>c. 



Ccelom 



Fig. 54. — Diagrammatic transverse section of a vertebrate 

 embryo (adapted from Minot) . 



