334 THE DEVELOPMENT OF THE CHICK 



rider in his saddle, the body of the rider being represented by 

 the heart, and his legs by the omphalomesenteric veins. On 

 each side of this posterior wall the parietal cavity communicates 

 with the coelome of the trunk. The floor of the parietal cavity 

 comprises two parts meeting at the head-fold, the anterior part 

 being composed of somatoj:)leure, and the posterior part of 

 splanchnopleure; the former is part of the definitive pericardial 

 wall, the latter, known as the precardial plate, is provisional 

 (Fig. 67). 



The lateral mesocardia also take part in bounding the parietal 

 cavity. It will be remembered that these arise as a fusion on 

 each side between the somatopleure and the primitive omphalo- 

 mesenteric veins, and that the ducts of Cuvier develop in them. 

 As the blastoderm is spread out flat at the time that they form, 

 they constitute at first a lateral boundary to the posterior part 

 of the parietal cavity; but as the embryo becomes separated from 

 the blastoderm they assume a frontal position between the sinus 

 venosus and body-wall, the original median face becoming dorsal 

 and the lateral face ventral. Thus they come to form a dorsal 

 wall for the posterior part of the parietal cavity (Fig. 119). The 

 communication of the parietal cavity with the coelome of the 

 trunk is thus divided into two, known respectively as the dorsal 

 parietal recess and the ventral parietal recess. The former is 

 a passageway above the lateral mesocardia, communicating in 

 front with the parietal (pericardial) cavity and behind with the 

 trunk cavity; the latter is a communication on each side of the 

 wall of the anterior intestinal portal ventral to the lateral meso- 

 cardia. 



The completion of the posterior wall of the pericardium is 

 brought about by the formation and development of the septum 

 transversum. 



Septum Transversum. The septum transversum arises from 

 three originally distinct parts, viz., (1) a median mass, (2) the 

 lateral mesocardia, and (3) lateral closing folds arising from 

 the body-wall between the umbilicus and the lateral mesocardia. 



1. The median mass proceeds from the ventral mesentery 

 of the fore-gut. The location of the heart and liver in the ventral 

 mesentery divides it in three parts, viz., (a) a superior part, 

 comprising the mesocardium and dorsal ligament of the liver 

 (gastrohepatic ligament), uniting the floor of the fore-gut and 



