CHANGES IN THE OVUM. 



87 



placental villosities. As we have already stated, these capillaries have 

 no direct communication with the maternal vessels, but after attaining 

 their finest dimensions pass into the veins which finally constitute the 

 UMBILICAL VEIN. This vessel, then, owes its origin to the capillary 

 radicles of the placental villosities; which radicles, by their union be- 

 tween the chorion and amnion, form a voluminous network whose rich- 

 ness is even greater than the arterial arborization. Two chief trunks 

 finally issue from this plexus, and these soon join to form a single trunk, 

 which accompanies the two corresponding arteries in the cord. On 

 reaching the umbilicus, this vessel, now the tujibilical van, bends forward 

 on the inner surface of the abdominal wall, where it is covered by peri- 

 toneum, and on gaining the liver enters that organ to open directly into 

 the vena portai. Owing to this junction, it happens that the two vessels 

 compose, in the interior of the liver, a single canal, from which proceed 

 the hepatic veins. In other animals than solipeds, this single canal sends 

 off a particular vessel of considerable size — the ductus venosus — that 

 passes directly into the posterior vena cava. The umbilical vein has no 

 valves. 



Besides these three principal vessels, the cord contains, in its amniotic 

 portion, the duct of the umbilical vesicle, the urachus, and the omphalo- 

 mesenteric vessels, as well as the extremity of the foetal intestine at an 

 early period. The urachus is an irregularly bulging canal, continued 

 from what is eventually the fundus of the bladder, and on reaching the 

 umbilical opening it passes between the chorion and the amnion to form 

 the allantois. 



After birth it rapidly contracts, especially at the fundus of the bladder, 

 until it is quite closed, and nothing is left but the fold of peritoneum 

 that sustained it, and which now becomes the middle ligament of the 

 bladder. It sometimes happens, however, with the foal, but more fre- 

 quently the calf, that it persists, the urine in this case escaping by the 

 umbilicus. 



The omphalo-meseiiteric vessels are an artery and a vein. The first is 

 given off from the anterior mesenteric artery, and passes to the amniotic 

 extremity of the umbilical vesicle \ while the vein arising from this vesi- 

 cle terminates in th^ vena portse. These two very thin vessels become 

 obliterated with the vesicle. 



DIFFERENCES. 



Rumijiants. 



In Ruminants, the two veins passing from the chorion remain separate 

 in the umbilical cord until they enter the umbilical ring, where they 

 become one vessel. There are, therefore, two veins and two arteries ; 

 the latter unite at the umbilicus, the resulting vessel entering the vena 

 cava and vena portae, between which it establishes a communication by 

 means of the ductus venosus. The chorion and the amnion being in 

 immediate contact over a wide surface, the umbilical vessels are re- 

 flected over the inner face of the first-named membrane on their leaving: 

 the amniotic sheath ; they do not have a fold of allantois, as in solipeds* 

 No traces of the omphalo-mesenteric vessels have been found. 



Fig. 

 In the Pig the arrangement of the cord is the same as in ruminants. 



