252 



STUDY OF PIG EMBRYOS. 



cavities do not show, the position of these vessels is marked by the darker line 

 of tissue. The origin of one of these intersegmental vessels from the dorsal aorta, 

 Ao, is indicated in the lower part of the figure. The Wolffian body, W.b', W.b", 

 extends from the level of the lungs and liver well down toward the pelvic end 

 of the embryo. Its ventral limit is marked by the body-cavity, Ccc, and it is, 

 of course, covered by a layer of mesothelium, msth, which here, as everywhere and 

 at all stages, forms the boundary of the ccelom. In the Wolffian body we dis- 

 tinguish readily numerous sections' of the epithelial Wolffian tubules, and toward 

 the ventral side of the organ the characteristic glomeruli, Glo. Between the glo- 

 meruli and the mesothelium there is a layer of mesenchyma, but between the tubules 

 there is little tissue, the intertubular spaces being almost entirely occupied by 

 sinusoids developed from the cardinal vein. The larger sinusoid or venous space, 

 V.msn, is due to the section of the venous trunk which joins the lower end of 

 the vena cava inferior, and is known as the sub-cardinal vein. In the upper part 

 of the figure we encounter a section of the descending aorta, Ao.d, and of the 

 lungs, Pul, or pulmonary anlage. The latter consists of a ring of entoderm bound- 

 ing the central cavity and enclosed by a thicker layer of mesenchyma, which, again, 

 is bounded by a layer of mesothelium. The space or ccelom about the lung is 

 shown in the figure to be continuous with the ccelom of the abdominal region. 

 On the ventral side we have the heart partly shown, the ventricle, Ven, being so 

 cut as to exhibit the trabecular structure of the network of the sinusoidal spaces. 

 The auricle, Au, is without sinusoids. The great venous trunk, vena hepatica 

 communis, V.h.c, opens into the auricle, the opening being guarded by two 

 valves, that on the dorsal side of the opening in the figure, V.s, being the left 

 valve. The vein receives blood from the liver, Li, and from the Wolffian bodies, 

 and it persists in the adult as the uppermost part of the vena cava inferior. The 

 duct-us venosus Arantii, which is so large in the human fetal liver, is less conspicu- 

 ous in the pig; the ductus is the venous trunk formed by the union of the portal 

 and umbilical veins within the liver; it joins the vena cava inferior to form the 

 vena hepatica communis. The liver, Li, consists of liver cells or hepatic cylinders 

 and numerous sinusoids of many diameters. On the lower side of the liver there 

 is a considerable accumulation of mesenchyma by which the liver is united on 

 the one end to the body-wall, Som, to the umbilical wall, Um.w', and to the, 

 mesentery by which the intestine is suspended from the liver. In this mesen- 

 chyma is lodged the gall-bladder, which is cut thrice. The reference line G.b 

 runs to the uppermost of the three sections. The diameter of the gall-bladder 

 is ^several times that of the entodermal intestine, and its lining epithelium is thicker 

 than any other epithelium of the embryo at this stage. The section of the bladder 

 nearest the portal vein, V.p, corresponds to the beginning of the ductus cysticu 

 Underneath the liver in the section of the mesentery is situated the portal vein, 

 V.p. From the mesentery extends out the intestine (duodenum). It is a somewhat 

 cylindrical tube which curves over ventralward and passes out through the opening 



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