CHICK EMBRYO OF EIGHTEEN PRIMITIVE SEGMENTS 



NotochorJ 

 Wephrotome 



Archenteron - 



Splanchnic 

 mesoderm 



Entoderrn 



Neural tube 



Mes. Segment 



Ectoderm 



mesoderm 



YolK 



with the mesonephros, and as the kidney of the adult (metanephros) is partly 

 developed as an outgrowth of the primary excretory duct, we may regard the 

 intermediate cell mass as the anlage of the urogenital glands and their ducts. They 

 are thus of mesodermal origin. 



Somatopleure and Splanchnopleure. In the embryo of seven primitive seg- 

 ments we saw that the mesoderm split laterally into two layers, the somatic 

 (dorsal) and the splanchnic (ventral) mesoderm. 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 epicardium and myocardium of the heart, visceral pleura of the 

 lungs, the mesenteries and 

 mesodermal layer of the gut. 

 The somatic mesoderm and 

 the ectoderm with the tissue 

 developed between them con- 

 stitute 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, termed the 

 Splanchnopleure. 



Ccelom. The cavity be- 

 tween 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. 52). Later, it is subdivided into the pericardial cavity 

 of the heart, the pleural cavity of the thorax and the peritoneal cavity of the 

 abdominal region. In the stages we have studied, the embryo is flattened on the 

 surface of the yolk and the somatopleure and Splanchnopleure do not meet ven- 

 trad. If this were the case we should have the structural relations as in Fig. 52 

 which is essentially the ground plan of the vertebrate body. 



Mesenchyma. In the sections through the head of this embryo and through 

 that of the preceding stage, we have found but three primitive segments present. 

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



\Coelom 



FIG. 52. Diagrammatic transverse section of a vertebrate 

 embryo (adapted from Minot). 



