54 THE STUDY OF CHICK EMBRYOS 



lengthwise in the furrow. These cords hollow out, grow caudad, and 

 become the primary excretory (mesonephric) ducts (Fig. 53). More caudally 

 the intermediate cell masses form the embryonic kidney, or mesonephros, 

 the tubules of which open into the primary excretory duct. Further 

 details concerning these provisional kidneys are given on pages 196-200. 

 Since the genital glands develop in connection with the mesonephros, 

 and the kidney of the adult (metanephros) is partly developed as an out- 

 growth of the primary excretory duct, the intermediate cell mass may be 

 regarded as the anlage of the uro genital glands and their ducts. These 

 structures are thus of mesodermal origin. 



Somatopleure and Splanchnopleure. In the embryo of seven primi- 

 tive segments the mesoderm was seen to split laterally into two layers, 

 the somatic (dorsal) and the splanchnic (ventral) mesoderm (Fig. 34). 

 These layers persist in the adult, the somatic mesoderm giving rise to the 

 pericardium of the heart, to the parietal pleura of the thorax, and to the 

 peritoneum of the abdomen, while the splanchnic layer forms the epicar- 

 dium and myocardium of the heart, the visceral pleura of the lungs, and 

 the mesenteries and mesodermal layer of the gut. The somatic mesoderm 

 and the ectoderm, with the tissue developed between 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 Splanchnopleure. 



Notochord Neural tube 



k ' 



Splanchnic 

 mesoderm 



Entoderm 



Codom 

 FIG. 54. Diagrammatic transverse section of a vertebrate embryo (adapted from Minot). 



Coelom. The cavity between the somatopleure and 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 ccelom. With the extension of the mesoderm, the ccelom 

 surrounds the heart and gut ventrally (Fig. 54). Later, it is subdivided 



