238 BODY-CAVITY 



Behind the heart the walls of the yolk-sac are nipped in, and come together, 

 before they are folded in from the front by extension of the mesial pericardial 

 cleft, so that here a ventral mesentery is produced, as well as a dorsal, and this 

 continues to be formed until the union of the lateral edges of the shield has 

 extended beyond the point where somatopleure and splanchnopleure are con- 

 tinuous with one another. This union primarily extends all round the front edge 

 of the shield ; when, therefore, the head-fold is formed, and the splanchnopleuric 

 layers unite with one another, a plate of mesoderm will extend right across 

 the body of the embryo below the gut and lateral ccelomic spaces, and behind 

 the pericardium. This is the septum transversum. 1 It forms a bridge of tissue 

 in which the allantoic and Cuverian veins pass inwards and join the vitelline, 

 to form the sinus venosus. It is at first set nearly transversely, but as the 

 pericardium extends backwards it becomes rotated, till it lies obliquely in an 

 antero-posterior and dorso-ventral plane. It forms the dorsal and posterior walls 



/.-' -,- f _._. t nolens aortie 



.* .'';'. post, meaocardium 

 pericardium 



\Y j "\ auricular portion of 



ant. wall of , ..--""" heart loop 



pericardium "T" ' ;j\ ... - 



_._ :..-^ lat. mesoca rdi /mi 



, ..- ..,,[;[<<( of Cuvier 



septum 

 transversum 



liver parenchyma 

 liver divert 



----umbilical rein 

 ^- vitelline vein 



mouth of yolk-xti*-.,. . 



sV cnslom 



FIG. 292. THE PEBICABDIAL, PLEURAL, AND ABDOMINAL PORTIONS OF THE CCELOM AND MESOCABDIA 

 IN A HUMAN EMBRVO OF 3 MM. LONG. (After His, from Kollmann.) 



of the pericardium, while in its anterior edge, which bounds the pleuro-pericardial 

 opening, lie the sinus venosus and the Cuverian veins, which sweep into it 

 from the somatopleure on each side. Between the dorsal surface of the septum 

 and the back wall of the body-cavity stretches the mesenteric partition, 

 containing the gut and its hepatic diverticulum, on each side of which the 

 vitelline veins extend forwards through its substance to the sinus venosus. The 

 septum transversum soon becomes greatly thickened by the extension into it 

 of the epithelial trabeculae of the liver parenchyma. These follow the 

 walls of the veins, which they surround, imbed, and break up into the sinusoids 

 already described (p. 174). The primary expansion takes place forwards, but 

 does not extend to the edge containing the sinus venosus. Later the trabeculee 

 grow up on each side to form lateral masses of liver-substance which project into 



1 The view adopted in the text regarding the formation of the septum transversum is not shared by 

 embryologists. "- ' " ^ .......... 



brought secondarily 



all embryologists. Many following Ravn consider that the somatopleure and splanchnopleure 

 rily into union by the enlargement of the omphalo-mesenteric vein. 



