TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 285 



accompanied by nerve-fibers. Below the aorta runs a ring of epithelium, (E, 

 representing the entoderm of the oesophagus, and farther ventralward a second 

 layer of epithelium, Tra, the entodermal lining of the trachea. Both of these 

 rings of epithelium are surrounded by somewhat condensed mesenchyma, the 

 differentiation of which about the oesophagus is more advanced than about the 

 trachea. Around the oesophagus next to the epithelium is a thin, looser layer of 

 mesenchyma, the anlage of the mesodermic portion of the future mucous membrane, 

 and perhaps also of the submucosa. Outside of this looser mesenchymal envelope 

 is a second denser layer in which the cells appear elongated, having begun their 

 differentiation into smooth muscle-cells. To the right and . left of the oesophagus 

 and at a slightly lower level lie the sections of the vagus nerves, the right nerve 

 being situated a little higher than the left. To the right and the left of the aorta 

 appear the very large posterior cardinal veins, card. From the sides of the trachea 

 project lobes of tissue which represent the anlages of the lungs. These lobes of 

 tissue are each covered by a layer of mesothelium, and protrude, as it were, into 

 the ccelom of the pleural cavities, ^leur. Farther to one side the ccelom, Coe, 

 of the abdominal cavity is also in part shown. It is bounded externally by the 

 body-wall, Som, of the embryo. Below the trachea in the median line is a small 

 blood-vessel, a section of the pulmonary vein. As regards the great nerve of the 

 limb, N.S, it must be remembered that it forms a portion of the brachial plexus 

 and is joined by other cervical nerves. From the voluminous trunk thus developed 

 there arise three principal branches; the first, xx, is at the base of the limb, is 

 small, and runs off dorsally. The other two represent a terminal forking of the 

 nerve-trunk, one, yy, running to the dorsal side of the limb, the other, zz, to 

 the ventral side. 



Section through the Stomach and Liver (Fig. 198). We now pass to a r section 

 well below the heart in order to study the characteristics of the Wolffian body, 

 stomach, and liver. At this level, as comparison with, figures 194 and 196 will 

 show, the body of the embryo has its greatest dimensions. The upper edge, Um, 

 of the umbilical cord appears in this section. The spinal cord with its ganglia 

 and nerves presents essentially the same features as in figure 196. The notochord, 

 Nch, forms a small circle in section and is surrounded by the anlage of an inter- 

 vertebral disc, which appears as an area relatively large, over which the mesen- 

 chymal cells are more crowded or condensed than elsewhere. At its peripliery the 

 anlage merges without divisional boundary into the surrounding niesencb' ui. 

 It is more expanded laterally than ventrally. In the median line belc noto- 



chord is the large dorsal aorta, Ao, which is formed by the union of the two 

 descending aortae shown in figure 196, and which extends h the abdominal 



region of the embryo to the pelvic region, where it fork- ;m the two allantoic 



arteries, which, passing on either side of the intestine, Lie their course al<ig 



the side of the internal allantois or future blade 1 , jiey reach the umbilicus, 



where they enter the umbilical cord to su^ * -a -embryonic or placental 



