EMBRYO OF 17 MM. 235 



tissue connected with its fellow across the median line of the embryo and pro- 

 jecting laterally far into the pleural cavity, PI. cos. Each lung consists chiefly 

 of a large accumulation of dense mesenchyma in which the epithelial bronchi, 

 bro, ramify. Each 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 every- 

 where bounded by mesothelium which persists throughout life, being known in 

 the adult as the pleural epithelium. 



Section through the Wolffian Body and Genital Gland. — The general charac- 

 teristics of the ectoderm, mesenchyma, and nervous system are nearly the same 

 as in the section last described. On one side the section shows a thickening of 

 the ectoderm, the anlage of a mammary gland, mam (compare page 249). 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 together. 

 On the median side of the organ appear the large glomeruli, Glo, and on their ven- 

 tral side we have the section of the longitudinal Wolffian duct. The tubules of 

 the Wolffian body are formed by a more or less nearly cuboidal epithelium, the 

 nuclei of which are decidedly larger than those of the mesenchymal cells. The 

 nuclei themselves stain deeply, have well-marked outlines, and very distinct 

 granules in their interior. The protoplasm of the cells also stains somewhat with 

 cochineal, carmine, hematoxylin, etc. There is very little mesenchyma in the 

 organ, but each tubule is closely invested by vascular endothelium; hence the 

 tubules are separated from one another only by blood spaces, which, morphologi- 

 cally speaking, are portions of the cavity of the cardinal vein. These blood spaces 

 are highly characteristic and are typical sinusoids. The intertubular circulation 

 of the Wolffian body is, so far as known, always sinusoidal. The aorta, Ao, is 

 seen in the figure to give off a small branch, art, which runs toward the Wolffian 

 body. There are numerous such branches, each one of which may be traced to a 

 glomerulus of the mesonephros. Each glomerulus has a capillary circulation, 

 and the blood on leaving the glomerulus is supposed to be emptied into the 

 venous sinusoids. More exact investigation of this point is needed. The meso- 

 nephros is covered by a layer of mesothelium, msth, underneath which is a thin 



