STUDY OF SECTIONS OF EMBRYO OF 20 MM. 317 



veins, card. These are now quite small vessels, the vena cava inferior having 

 become the main channel for the return of the blood from the abdominal region to 

 the heart. The two cardinal veins are not quite symmetrically placed, that on the 

 left side lying a little lower than that on the right. Between them is situated 

 the median aorta, Ao, with a relatively thick and well-developed muscular coat, 

 the deeper staining of which makes it conspicuous even with low powers. The 

 (esophagus, (E, and trachea, Tra, are not in the median line, but are both dis- 

 placed toward the right of the embryo.. As compared with earlier stages, both 

 structures show an advance, first, by the growth of the entoderm, and, second, by 

 the differentiation of the surrounding mesenchyma. In both oesophagus and trachea 

 the entoderm is a ring of cylinder epithelium, the tracheal ring being much larger 

 than the oesophageal. The mesenchyma about the oesophagus forms two distinct 

 layers, an inner looser layer and an outer denser muscular layer. Around the 

 trachea the. mesoderm is much condensed. On the dorsal side of the trachea the 

 cells form next to the epithelium a special layer characterized by the elongated 

 form of the cells. Between the oesophagus and trachea are situated the vagus nerves, 

 that of the right side, N.io, occupying a higher position than that on the left, 

 so that the nerves are not symmetrically placed. The cardinal veins, the aorta, 

 the oesophagus, the vagus nerve, and the trachea are all imbedded in mesenchyma, 

 which, together- with these structures, forms the so-called mediastinum by which the 

 right and left pulmonary cavities, Pl.d, Pl.s, are separated from one another. On 

 its ventral side the mediastinum joins on to the veins entering the heart. On either 

 side of the mediastinum at the level of the trachea may be seen the projecting 

 lung. That on the left side shows clearly the division of the organ into a dorsal 

 lobe, Lu.d, and a ventral lobe, Lu.v. Each lung consists at this stage chiefly of 

 mesenchymal tissue and is covered by a layer of mesothelium which forms the 

 boundary of the pleural ccelom. Within the mesenchyma appear several sections 

 of the branches of the entodermal bronchi. Each bronchus is lined at this stage 

 by a rather thick entodermal layer of cylinder cells. The union of the lung with 

 the mediastinum constitutes the so-called root of the lung. In the root of the lung 

 is seen the small pulmonary artery, A.pul. The two arteries join a little nearer the 

 head and on the left side of the embryo to form a single trunk, the main pul- 

 monary artery. Originally the pulmonary arteries arise symmetrically as branches 

 from the fifth aortic arches. They soon unite, however, throughout the greater 

 part of their extent, forming a single vessel. The two arteries shown in our figure 

 represent the two original symmetrical vessels where they are about to enter the 

 lungs. On the ventral side of the section various cardiac structures are shown, 

 but so cut that the picture is not very instructive. It will suffice to refer to the 

 explanation of the figure for the identification of the parts. 



Sections through the Miillerian Ducts (Fig. 215). The female or Miillerian 

 ducts are remarkable for their late development. In the 12 mm. pig the 

 small funnel-shaped in aginations of the mesothelium, which represent the first 



