322 STUDY OF PIG EMBRYOS. 



the mesothelium, so that we may say that two layers of mesenchyma are already 

 imperfectly differentiated within the rib. The denser layer next the mesothelium 

 is destined to become still more marked and to transform itself into the connective- 

 tissue layer of the peritoneum. With the overlying mesothelium it develops into 

 the peritoneal membrane of descriptive anatomy. 



Sagittal Section through the Right Lung and Kidney (Fig. 218). The lungs 

 occupy a position in the upper part of the figure and are easily recognized by the 

 conspicuous- entodermal bronchi, bro, which resemble in microscopic structure the 

 bronchi of earlier stages. The branches are widely separated from one another by 

 the voluminous mesenchyma of the organ. . The lung is covered by mesothelium, 

 msth, and projects into the pleural cavity, Pleu.c, which is lined by a continuation 

 of the mesothelium of the lung itself. The pleural cavity can be followed down- 

 ward past the Wolffian body, W.b', and liver, and from there past the genital 

 gland, Gen, and so on to the lowest part of the abdominal cavity, Ab.cce. The 

 pleural cavity at this stage is entirely separated from the pericardial, but it is 

 still directly continuous with the abdominal cavity. On the ventral side (in the 

 figure, to the right) of the pleural cavity are the great veins, the common cardinal, 

 C.C, descending from above, and the ductus venosus, Du.v, rising from below. The 

 pleural cavity is separated from the common cardinal by a lamina of the mesoderm, 

 x, and from the ductus venosus by a similar but thinner lamina, Y. Both laminae 

 are bounded on the tdeural side by the mesothelium, and on the venous side by 

 the endothelium of the vessel. The opening of the veins into the right auricle, 

 Au.d, does not appear in^this section, though a small bit of the left valve, v.s, 

 which guards this opening is shown. The Wolffian body is divided into two parts, 

 an upper, W.b', on a level with the liver, and a lower, W.b", toward the pelvic 

 end of the abdomen. The lower part is larger than the upper. The two parts are 

 separated from one another chiefly by the mesonephric vein, V.msn, which is the 

 principal vessel to take the blood from the Wolffian body. It delivers the blood to 

 the lower end of the vena cava inferior. The separation of the two parts of the 

 Wolffian body is, however, further accented by the position of the genital gland, 

 Gen, and of the kidney, Ki. The structure of the latter organ does not differ much 

 from that of earlier stages, but the diameter of the tubules has increased, and there 

 has been an advance in the differentiation of the convoluted tubules and of the 

 glomeruli. The genital gland (testis) is remarkable for its large size. It is covered 

 by a layer of mesothelium, underneath which is a rather broad layer of elongated 



FIG. 218. PIG, 20.0 MM. SAGITTAL SERIES 60, SECTION 213. 



Ab.cce, Abdominal ccelom. All.vi, Mesothelial villi of the allantois. Au.d, Right auricle, bro, Entodermal 

 bronchus. C.C, Common cardinal. Cae', Cce", Ccelom. Diaph', Diaph", Diaphragm. Du.v, Ductus 

 venosus. G.bl, Gall-bladder. Gen, Genital gland. In', In", In'", Intestine. Ki, Kidney. Li, Liver. 

 Mes, Mesenchyma. msth, Pleural mesothelium. P.cce, Pleural ccelom. Pleu.c, Pleural cavity. Ve. hep, 

 Ve.hep', Hepatic veins. V.msn, Vena mesonephrica. v.s, Valvula sinistra. W.b', W.b", Wolffian body. 

 x, Partition separating the pleural cavity from the duct of Cuvier. Y, Partition separating the pleural cavity 

 from the ductus venosus. X 22 diams. 



