3354 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 ccelome of the trunk. The floor of the parietal cavity 
comprises two parts meeting at the head-fold, the anterior part 
being composed of somatopleure, 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 criginal 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 (38) 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 superior part, 
comprising the mesocardium and dorsal ligament of the liver 
(gastrohepatie ligament), uniting the floor of the fore-gut and 
