TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 201 



ramus. The trunk, N, which runs toward the limb is the ventral ramus. Below 

 the spinal cord is the notochord, Nch, which is completely surrounded by a very 

 dense mass of mesenchymal cells, Vert, the anlage of the body of a vertebra- 

 Lower down in the section are the two descending aorta? , Ao, which are at this 

 point just uniting to form the single median dorsal aorta. Below the aorta runs a 

 ring of epithelium, CE, representing the entoderm of the oesophagus, and further 

 ventralwards a second layer of epithelium, Tra, the entodermal lining of the 

 trachea. Both of these rings of epithelium are surrounded by somewhat con- 

 densed mesenchyma, the differentiation of which about the oesophagus is more 

 advanced than about the trachea. Around the oesophagus next to the epithe- 

 lium 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 ap- 

 pear elongated, having begun their differentiation into smooth muscle cells. 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 meso- 

 thelium, and protrude, as it were, into the ccelom of the pleural cavities, Pleur. 

 Further to one side the ccelom, Car, of the abdominal cavity is also in part shown. 

 It is bounded externally by the body- wall, Som, of the embryo. To the right and 

 the left of the oesophagus lie the sections of the vagus nerve, the right nerve a 

 little higher up than the left. Below the trachea in the median line is a small 

 blood-vessel, a section of the pulmonary vein. Above the aorta, on the right and 

 left, lies a small cluster of rather darkly stained cells, intermingled with which 

 one can make out with a higher power the future nerve-fibers. These structures 

 are portions of the sympathetic nervous system. As regards the great nerve of the 

 limb, N.8, 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 devel- 

 oped 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. — We now pass to a 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 118 and 120 will show, the body 

 of the embryo has its greatest dimensions. The upper edge, Urn, of the umbilical 

 cord appears in this section. The spinal cord with its ganglia and nerves pre- 

 sents essentially the same features as in figure 120. The notochord, Nch, 

 forms a small circle in section and is surrounded by the anlage of the body 



