226 STUDY OF PIG EMBRYOS. 



tion with the liver which renders it possible for the blood of the right subeardinal 

 vein to pass through the blood spaces of the liver directly to the heart. This 

 makes a very direct channel, a more direct one than existed previously, when the 

 blood from the subeardinal came to join that of the cardinal, passing up to the 

 duct of Cuvier and then back to the heart. The new channel through the liver 

 rapidly enlarges and becomes recognizable as the vena cava inferior. This im- 

 portant venous trunk is a combined vessel, comprising, first, a part of the sinus 

 venosus of the heart; second, the ductus venosus of the liver; third, a large 

 channel developed from the sinusoids of the liver; fourth, the upper part of the 

 right subeardinal vein; and, fifth, the lower part of the right cardinal. The vena 

 cava inferior has already been developed in the pig embryo of 9 mm. Be- 

 tween the two Wolffian bodies hangs down the large intestine, Red, suspended 

 by its mesentery in the median line. The entodermal portion is a very small 

 circle of epithelium with an extremely minute lumen, which in the section is 

 scarcely larger than a single nucleus. The mesentery and intestine are covered 

 by a well-defined mesothelium and have a considerable amount of mesenchyma, 

 in which there is no distinct histological differentiation beyond the presence of a 

 number of small blood-vessels. At this stage the large intestine runs nearly in 

 the median plane to the pelvic end of the body. In the opposite direction, to- 

 ward the head, it bends to the left of the embryo, making a loop which passes over 

 into the end of the ileum. The ileum forms the continuation of the loop and ex- 

 tends into the coelom at the base of the umbilical cord. There it bends back and 

 returns toward the dorsal side of the embryo to pass over into the duodenum and 

 join the stomach. Owing to the fact that the small intestine extends into the 

 extra-embryonic ccelom of the umbilical cord, there makes a loop, and returns to 

 the embryonic region, we get typically a double section of the intestine as shown 

 in the figure, one of each limb of the loop. The entoderm, In, in these loops forms 

 a small ring, which, however, is much larger than the entodermal ring of the large 

 intestine at this stage. Each loop contains a large amount of mesenchyma, mes, 

 the cells of which are somewhat crowded, so that the tissue appears dark in the 

 stained section. The boundary between the body of the embryo and the tissue 

 of the umbilical cord is marked by the position of the two umbilical veins, that of 

 the left side, V. U.S, being very much larger than that of the right side, V. U. D. 

 By following down the somatopleure, Som, of the embryo, it will be seen that 

 these veins are lodged therein, and that the continuation of the somatopleure 

 beyond these veins forms the substance of the umbilical cord. The limb-bud, 

 P. L, is a large mass of rather dense mesenchyma, entirely without muscles or 

 nerves and covered by ectoderm. At the edge of the limb-bud the ectoderm 

 shows a special thickening, F. The theory has been advanced that this thicken- 



