EMBRYO OP p MM. 221 



eruli and the .mesothelium there is a layer of mesenchyma, but, between the 

 tubules there is little tissue, the intertubular spaces being almost entirely occu- 

 pied 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 mesonephric 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. It consists of a ring of entoderm 

 bounding the central cavity and enclosed by a thicker layer or mesenchyma, 

 which, again, is bounded by a layer of mesothelium. The space or coelom about 

 the lung is shown in the figure to be continuous with the coelom 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 or 

 ductus venosus, D. ven, 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 ductus venosus receives its blood-supply from the liver, Li, 

 which 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 f , and to the mesentery by which the intestine is sus- 

 pended from the liver. In this mesenchyma are lodged three spaces bounded 

 by entoderm, the uppermost of which is indicated by the reference line, A . It is 

 just at this point that the development of the gall-bladder and the intestine 

 takes place, and the exact identity of the three entodermal structures just re- 

 ferred to has not yet been satisfactorily worked out. Underneath the liver in 

 the section of the mesentery is situated the portal vein, P. v. From the mesen- 

 tery extends out the intestine (duodenum). It is a somewhat cylindrical tube 

 which curves over ventral wards and passes out through the opening of the umbil- 

 icus. It consists of a very small tube of entoderm, Ent, with only a small inter- 

 nal cavity (compare Fig. 132, Red.). The thickness of the intestinal wall is due 

 chiefly to the considerable development of the mesenchyma. The external 

 covering of the intestine is a layer of mesothelium which becomes the peritoneal 

 epithelium of the adult. In the tissue of the organ we distinguish the narrow 

 vitelline artery, Art. v. The umbilical opening is quite wide and is bounded both 

 above and below by a prolongation, Um. w f , Um. w" , of the somatopleure of the 

 embryo. The wall on the upper side is much thicker than on the lower. The 

 opening of the umbilicus is very wide. It is partly occupied by the duodenum. 

 Appended to the inferior wall of .'.if umbilu : is the allantois, All, which 

 arises from the enlarged caudal ^nd (cloaca), C lot of the intestine. It passes 



