TRANSVESSE SECTIONS OF A TEN MM. PIG EMBRYO 



131 



side through the airio-ventricular foramen. Between these openings is the endocardial cushion, 

 which in part forms the anlages of the tricuspid and bicuspid valves. The atria are marked off 

 externally from the ventricles by the coronary sulcus. Between the two venticles is the inter- 

 ventricular septum. The ventricular walls are thick and spongy, forming a network of muscu- 

 lar cords or trabecule surrounded by blood spaces or sinusoids. The trabeculae are composed 

 of muscle cells, which later become striated and constitute the myocardium. They are sur- 

 rounded by an endothelial layer, the endocardium. The mammalian heart receives all its 

 nourishment from the blood circulating in the sinusoids until later, when the coronary vessels 

 of the heart wall are developed. The heart is surrounded by a layer of mesothelium, the 

 epicardium, which is continuous with the pericardial mesotheUum lining the body wall. 



Section through the Liver and Upper Limb Buds (Fig. 136). — The section is 

 marked by the presence of the upper limb buds, the liver, and the bifurcation of the trachea to 

 form the primary bronchi of the lungs. The limb buds are composed of dense undifferentiated 

 rriesenchyme surrounded by ectoderm which is thickened at their tips. The seventh pair 

 of cervical ganglia and nerves are cut lengthwise showing the spindle-shaped ganglia with the 

 dorsal root fibers taking origin from their cells. The ventral root fibers arise from the ventral 



Foramen male 

 R. atrium 



R. atrio-ventricidar foramen 



R. ventricle- 



Interventricular septum 



L. alrium 



■Septum I 



Endocardial cushion 



L. atrio-venlricular foramen 



L. ventricle 



Fig. 135. — Transverse section through the foramen ovale 



X 22.5. 



of the heart in a 10 mm. pig embryo. 



cells of the mantle layer and join the dorsal root to form the nerve trunk. On the right side 

 a short dorsal ramus supplies the anlage of the dorsal muscle mass. The much larger ventral 

 ramus unites with those of other nerves to form the brachial plexus. 



The descending aortce have now fused and the seventh pair of dorsal intersegmental arter- 

 ies arise from the dorsal aorta. From these intersegmental arteries the subclavian arteries are 

 given off two sections caudad in the series. Lateral to the aorta are the posterior cardinal 

 veins. The esophagus, ventral to the aorta, shows a very small lumen, while that of the 

 trachea is large and continued into the bronchi on either side. Adjacent to the esophagus are 

 the cut vagus nerves. The lung anlages project laterally into the crescentic pleural cavities, of 

 which the left is separated from the peritoneal cavity by the septum transversum. 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 

 penetrating the septum transversum from the liver to the heart is the proximal portion of the 

 inferior vena cava, originally the right vitelline vein. Ventral to the bronchi may be seen sec- 

 tions of the pulmonary veins. 



