246 STUDY OF PIG EMBRYOS. 



Immediately around the notochord the spaces occupied by the cells are the 

 largest and the capsules most distinct, the nuclei most altered. Proceeding 

 toward the periphery of the cartilage, the cells appear in successively earlier and 

 earlier stages, until at the very periphery we have normal nuclei and a transition 

 to mesenchyma. The cells of the notochord are beginning to degenerate, and in 

 place of the notochordal sheath there seems to be only a space between the noto- 

 chordal cells and the vertebral cartilage. Immediately below the vertebra are the 

 conspicuous anlages of the sympathetic system, Sym. They overlie the sections 

 of the posterior cardinal 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 sym- 

 metrically 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 oesophagus, ffi, and trachea, Tra, are not in the median 

 line, but are both displaced toward the right of the embryo. As compared with 

 earlier stages, both structures show an advance, first, by the growth of the ento- 

 derm ; 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 . lo, 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, 

 PI. d, PI. 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 a projecting line. 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 

 coelom. Within the mesenchyma appear several sections of the branches of the 

 entodermal bronchi. Each bronchus is lined at this stage by a rather thick ento- 

 dermal layer of cylinder cells. The union of the lung with the mediastinum con- 

 stitutes the so-called root of the lung. In the root of the lung is seen the small 



