100 THE EARLY DEVELOPMENT OE MAMMALS. 



which the upper is connected with the arteries, the lower with the veins. The 

 change from the earlier condition to the later has still to be followed. 



According to Van Beneden's recent researches on the rabbit, the arrange- 

 ment of the main vessels in the area vasculosa at a later stage is quite different. 

 The sinus terminalis forms a complete ring (Fig. 45), and is connected with the 

 arterial system by a single trunk, which corresponds to the left omphalic artery 

 of the bird. For some time the connection between the embryonic arteries and 

 the area vasculosa is entirely through capillaries, and the arterial trunk on the 

 vascular area does not appear in the rabbit for several days. There are two 

 veins, one arising from each side of the body and passing out on to the area vascu- 

 losa over the back of the embryo; they are the two large upper vessels in the 

 figure. 



The Liver. 



When the omphalo-mesaraic veins, the first large veins to appear, are devel- 

 oped, they are situated in the splanchnopleure and join the heart. They are of 

 such large size as to cause a projection into the ccelom. This projection is the 

 septum transversum (p. 82). As shown in the diagram (Fig. 43), the entoderm 

 of the digestive canal of the head of the embryo passes over behind the pericardial 

 cavity and behind the septum transversum into the yolk-sac. Out of the ento- 

 derm covering the septum transversum on its caudal side, the anlage of the liver 

 is developed. This anlage is produced by a rapid proliferation of the entodermal 

 cells, and they grow toward the space occupied by the omphalo-mesaraic veins. 

 An intergrowth of the liver cells and of the endothelium of the veins takes place. 

 The cavity of the veins becomes subdivided into smaller blood channels which 

 we call sinusoids to distinguish them from capillary vessels. The liver cells 

 arrange themselves in the form of cords which are termed the hepatic cylinders. 

 Each hepatic cylinder is closely invested by the venous endothelium. The liver 

 consists at first only of hepatic and endothelial cells and is situated in the septum 

 transversum. 



When the liver becomes larger, it protrudes from the septum transversum, 

 but does not separate from it, so that in the adult the liver is always found at- 

 tached to the diaphragm, which is merely the modified septum transversum. 



The Oral and Anal Plates. 



These two structures resemble one another. Each occupies a small area 

 and is formed by the intimate union of the entoderm with the ectoderm. When 

 the union is first formed the two layers are distinct, but they soon fuse, so that no 

 boundary can be recognized between them. I 'ltimately both plates break down. 



