TRANSVERSE SECTIONS OF A TEN MM. PIG EMBRYO 



135 



turn transfer sum. The liver, with its fine network of trabeculae and sinusoids, is large and 

 nearly fills the peritoneal, or abdominal cavity. The liver cords are composed of liver 

 cells surrounded by the endothelium of the sinusoids. Red blood cells are developed in 

 the liver at this stage. The large vein, from the liver to the heart, penetrating the sep- 

 tum transversum is the proximal portion of the inferior vena cava, originally the right 

 vitelline vein. Ventral to the bronchi may be seen sections of the pulmonary veins. 



Section through Lung Buds, Cranial to Stomach (Fig. 137). The lungs are sectioned 

 through their caudal ends and the esophagus is just beginning to dilate into the stomach. 

 On either side of the circular dorsal aorta are the mesonephroi, while dorso-laterally are 

 sympathetic ganglia. The pleural cavities now communicate freely on both sides with the 

 peritoneal cavity. A section of the lesser peritoneal sac appears as a crescent-shaped slit at 

 the right of the esophagus. In the right dorsal lobe of the liver is located the inferior 

 vena cava. Near the median plane ventral to the lesser sac is the large ductus venosus. 



Spinal cord, 



Notochord 



Dorsal aorta 



Plica vena cava 



Inferior vena cava 

 lesser omentum 



Spinal ganglion 



Base of upper limb 



Glomerulns of 

 mesonephros 



Greater omentum 

 Stomach 



Dorsal lobe of liver 

 Ductus venosus 



Ventral lobe of liver 



Ventral attachment of live* 



FIG. 1 38. Transverse section through the stomach and liver of a i o mm. pig embryo. X 22.5 . 



Section through the Stomach and Liver (Fig. 138). Prominent in the body cavity 

 are the mesonephroi and liver lobes. The mesonephroi show sections of coiled tubules 

 lined with cuboidal epithelium. Glomeruli of the renal corpuscles are median in position 

 and develop as knots of small arteries which grow into the ends of the tubules. The thick- 

 ened epithelium along the median and ventral surface of the mesonephros is the anlage of 

 the genital gland. The body wall is thin and lined with mesothelium continuous with that 

 which covers the mesenteries and organs. The mesothelial layer becomes the epithelium 

 of the adult peritoneum, mesenteries, and serous layer of the viscera. The stomach lies on 



