TRANSVERSE SECTIONS OF A TEN MM. PIG EMBRYO 



139 



cords or trabeculce surrounded by blood spaces or sinusoids. The trabeculae are composed of 

 muscle cells, which later become striated and constitute the myocardium. They are surrounded 

 by an endothelial layer, the endocardium. From the blood circulating in the sinusoids the 

 mammalian heart receives all its nourishment until, later, 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 mesothelium lining the body wall. 



Section through the Liver and Upper Limb Buds (Fig. 131). 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 

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



Spinal 'yanqlfon 



<Spmal nerve 



Post, cardinal 

 vein 

 Mesonephros' 



Pleural cavity 

 Upper limb bud 



Liver 



Spinal cord 



nfotb chord 



..Descending 

 aorta. 

 Esophagus 



Bifurcation 

 of trachea 



Inf. vena 

 cava. 



. 131. Transverse section through the liver and upper limb buds of a 10 mm. pig embryo at the level 

 of the bifurcation of the trachea. X 22.5. Inf. vena cava, inferior vena cava. 



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 

 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 aorta have now fused and the seventh pair of dorsal intersegmental arteries 

 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. The lung anlages project laterally into 

 the crescentic pleural cavities, of which the left is separated from the peritoneal cavity by the 



