1 14 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



Transverse Section through the Heart (Fig. 104.) The section passes through the 

 bases of the upper limb buds. Lateral to the descending aortae are the common cardinal veins. 

 The right common cardinal opens into the sinus venosus which in turn empties into the right 

 atrium, its opening being guarded by the two valves of the sinus venosus. The trachea has now 

 separated from the esophagus and lies ventral to it. Both trachea and esophagus are sur- 

 rounded by a condensation of mesenchyme. The myocardium of the ventricles has formed 

 a spongy layer much thicker than that of the atrial wall. An incomplete interventricular 

 septum leaves the ventricles in communication dorsad. The septum primum is complete in 

 this section but higher up in the series there is an inter atrial foramen (see Fig. 96). The for- 

 amen ovale is not yet formed. 



SpinaJ chord 

 Notochord-. 



Ant cardinaJ 

 vein 



Ratrium 



SomaTopleure 



0/fachry 



Descending 

 aorTa. 



Pericardial 

 cavity 



Bui bus cordi's 



Telencephalon 



FIG. 103. Transverse section through the bulbus cordis and olfactory pits of a 6 mm. pig embryo. 



X 26.5. 



Transverse Section through the Lung Buds and Septum Transversum (Fig. 

 105). The tips of the ventricles lying in the pericardial cavity still show in this section. Dor- 

 sally the pericardial cavity has given place to the pleura-peritoneal cavity. Into this cavity 

 project ventrad the Wolffian ridges 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 mesen- 

 chyma. This mesenchyma is continuous with that of the somatopleure, and forms a complete 

 transverse septum ventrally between the liver and heart. This is the septum transversum 

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

 diaphragm. Passing through the septum are the two proximal trunks of the vitelline veins. 

 Projecting 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 end of the lesser peritoneal sac. 



