TRANSVERSE SECTIONS OF A SIX MM. PIG EMBRYO 



109 



chyme. The myocardium of the ventricles has formed a spongy layer, much thicker than 

 that of the atrial wall. An incomplete iiiterventricular septum leaves the ventricles in 

 communication dorsad. The septum primum is complete in this section, but higher up in 

 the series there is an interatrial foramen (cf. Fig. 101). The foramen ovale is hardly 

 formed. 



Section through the Lung Buds and Septum Transversum (Fig. no). The section 

 passes through the bases of the upper limb buds. The tips of the ventricles, lying in the 

 pericardia! cavity, still show in this section. Dorsally, the pericardia! cavity has given 

 place to the pleura-peritoneal cavity. Projecting ventrad into this cavity are the meso- 

 nephric (Wolffian) folds in which the posterior cardinal veins partly lie. Into the floor of the 

 pleuro-peritoneal cavities bulge the dorsal lobes of the liver, embedded in mesenchyma. 

 This mesenchyma is continuous with that of the somatopleure, and forms a complete trans- 

 verse septum ventrally between the liver and heart. This is the septum transversum which 



Spinal ganglion 

 Spinal nerve 



Descending aorta 



Pieuro-peritoncal cavity' 



R. lung bud' 



R. vitdline vein 



Septum transversum 

 Piricardial cavil 



R. ventricli 



Spinal cord 



Upper limb bud 

 Post, cardinal vein 



Esophagus 

 Dorsal lobe of liver 



Lesser sac 



L. vitelline vein 



L. ventricle 



FIG. 1 10. Tranverse section through the right lung bud and septum transversum of a 6 mm. 



pig embryo. X 26.5. 



takes part in forming the ligaments of the liver and is the anlage of a portion of the dia- 

 phragm. The two proximal trunks of the vitelline veins pass through the septum. Project- 

 ing laterally into the pleuro-peritoneal cavities are ridges of mesenchyma covered by 

 splanchnic mesoderm in which the lungs develop as lateral buds from the caudal end 

 of the trachea. The right lung bud is shown in the figure. Between the esophagus and 

 the lung is a crescent-shaped cavity, the cranial end of the lesser peritoneal sac. 



Section through the Stomach (Fig. in). The section passes through the upper 

 limb buds and just caudal to the point at which the descending aortae unite to form the 

 median dorsal aorta. As the liver develops in early stages, it comes into relation with the 

 plica tenet cowz.along the dorsal body wall at the right side of the dorsal mesogastrium. 

 The space between the liver and plica to the right, and the stomach and its omenta to the 

 left, is a caudal continuation of the lesser peritoneal sac. The dorsal wall of the stomach 



