306 STUDY OF PIG EMBRYOS. 



sympathetic chain, Sym, which consists partly of nerve-fibers, partly of ganglion 

 cells which have migrated along the nerve and taken up their position at its end. 

 These cells are easily recognized by their very dark staining. Their nuclei are a 

 little lighter than those of the neighboring mesenchymal cells, but the cells, owing 

 to -their deep coloration, are conspicuous even when the section is examined only 

 with the low power. The sympathetic anlage comes in close contact with a por- 

 tion of the cardinal vein, card, near the aorta. The main nerve-trunk, R.V, con- 

 tinues obliquely downward and presently forks into an upper and a lower branch. 

 The cardinal veins, card, lie on either side of the aorta, but they are almost 

 completely obliterated by the ingrowth of the Wolffian tubules, which subdivide 

 the vein into numerous smaller channels or sinusoids. The section also shows 

 two branches, Ve' ', and Ve", of the subclavian vein. The identity of these branches 

 has not yet been determined. Beneath the aorta, Ao, follows the oesophagus, (E, 

 the lumen of which is much smaller than that of the aorta. Its epithelium has 

 the general characteristics of the epithelial entoderm at this stage, being a rather 

 thick cylinder epithelium. As above mentioned, the differentiation of the mucous 

 and muscular layers of the oesophagus shows clearly. Below the oesophagus lie 

 the two large vagus nerves, N.io, and then follow the sections of the two lungs, 

 Lu. Each lung is a lobe of tissue connected with its fellow across the median 

 line of the embryo and projecting laterally far into the pleural cavity, Pl.cce. 

 The lung consists chiefly of a large accumulation of dense mesenchyma in which 

 the epithelial bronchi, bro, ramify. Every bronchus has a central lumen and its 

 walls are formed by a moderately thick layer of cylinder entodermal cells. The 

 surface of each lung is covered by mesothelium, which is shown as a distinct line 

 in the engraving. The mesothelium can be followed to the root of the lung, 

 where it is reflected on to the outer wall of the pleural chamber. The pleural 

 cavity, Pl.cce, is thus everywhere bounded by mesothelium which persists through- 

 out life, being known in the adult as the pleural epithelium. 



Section through the Wolffian Body and Genital Gland (Fig. 209). The gen- 

 eral characteristics of the ectoderm, mesenchyma, and nervous system are nearly 

 the same as in the section last described. On one side the section shows a thick- 

 ening of the ectoderm, the anlage of a mammary gland, mam (compare page 320). 

 The branches of the nerves are not so well shown in this section as in the previous 

 one. The level of our section corresponds to the lower end of the vena cava 

 inferior, which is marked at this stage by the two large mesonephric veins, V.msn, 

 which come from the Wolffian bodies and by their union constitute the lower end 

 of the vena cava. The mesonephric veins are, strictly speaking, portions thereof. 

 The Wolffian bodies are the most conspicuous structures shown in the section. 

 They consist chiefly of a great number of tubules, W.t, very much crowded to- 

 gether. On the median side of the organ appear the large glomeruli, Glo, and- on 

 their ventral side we have the section of the longitudinal Wolffian duct, W.D. 

 The tubules of the Wolffian body are formed by a more or less nearly cuboidal 



