TRANSVERSE SECTIONS OF A SIX MM. PIG EMBRYO 



107 



portion of the spinal cord, the notochord just ventral to it, the descending aorta, and ventro- 

 lateral to them the anterior cardinal veins. The nasopharynx now is small with a vertical 

 groove in its floor. This is the tracheal groove and more caudad it will become the cavity of 

 the trachea. The bulbus cordis lies in the large pericardial cavity. On either side the section 

 cuts through the cephalic portions of the atria. These will become larger as we go caudad in 

 the series. 



Transverse Section through the Heart (Fig. 109). — 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 entrance of the left common cardinal into the sinus venosus is some- 

 what more caudad in the series. The trachea has now separated from the esophagus and lies 

 ventral to it. Both trachea and esophagus are surrounded by a condensation of mesenchyme. 

 The myocardium of the ventricles has formed a spongy layer much thicker than that of the 



Spinal ganjion 

 Spinal ntr c 



Descending aorta- 



Pleuro-peritoneal cavily — /-<■ 

 R. lung hud- 

 R. vitelline vein 



Septum transversiim 

 Pericardial cavily 



R. ventriele 



Spifi 7/ cord 



f^- — Upper limb hid 



Post, cardinal vein 



Esophagus 

 Dorsal lobe of liver 



Lesser sac 



L. vitelline vein 



L. ventricle 



Tig. 110. — Transverse section through the right lung bud and septum transversum of a 6 mm. pig 



embryo. X 26.5. 



atrial wall. An incomplete intehentricular 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. 



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

 110). — The section passes through the bases of the upper limb buds. The tips of the ventricles 

 lying in the pericardial cavity still show in this section. DorsaUy the pericardial cavity has 

 given place to the pleuro-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 

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

 The two proximal trunks of the vitelline veins pass through the septum. Projecting laterally 



