190 



THE ENTODERMAL CANAL AND ITS DERIVATIVES 



Dorsal to the gut the splanchnic mesoderm of each side is folded together in 

 the median sagittal plane and constitutes the dorsal mesentery which extends to 

 the caudal end of the digestive canal (Figs. 182 and 183 C). This suspends the 

 stomach and intestine from the dorsal body wall and is divided into the dorsal 

 mesogastrium or greater omentum of the stomach; the mesoduodenum, the mesen- 

 tery proper of the small intestine, the mesocolon and the mesorectum. 



The covering layers of the viscera, of the mesenteries and of the body wall, 

 are continuous with each other and consist of a mesothelium overlying connective 

 tissue. They are derived from the somatic and splanchnic layers of mesoderm. 



Bulbus cordis Dorsal mesocardium 



Pericardial cavity 



Somatopleure 



Septum transversum 



Liver trabecules 

 Hepic diverticulum 



Yolk-stalk 



Sinus venosus 



Lateral mesocardium 

 Common cardinal vein 



Umbilical vein 



Vitelline vein overlying 

 stomach 



Pleuro-peritoneal canal 



Peritoneal cavity 



Fig. 184. — Reconstruction near median sagittal plane of a 3 mm. human embryo, showing the body 

 cavities and septum transversum (Kollmann's Handatlas). 



The primitive ccelom lies approximately in one plane, as in Fig. 181. With 

 the development of the head-fold and the ventral flexion and fusion of the heart 

 tubes, the pericardial cavity is bent ventrad and enlarged. The ventral meso- 

 cardium attaching the heart to the ventral body wall disappears and the right 

 and left limbs of the U-shaped cavity become confluent ventral to the heart. 

 The result is a single large pericardial chamber, the long axis of which now lies 

 in a dorso-ventral plane nearly at right angles to the plane of the pleuro-peri- 

 toneal cavities, and connected with them dorsally by the right and left pleuro- 

 peritoneal canals (Fig. 184). 



The division of the primitive ccelom into separate cavities is accomplished by 



