DEVELOPMENT. 



195 



FIG. 67. 



Diagram of the Embryo and its 

 Vessels, showing the circula- 

 tion of the umhilical vesicle, 

 and also that of the allantois, 

 beginning to be formed* 



surface of the vitellus. and as the folds of the embryo occur the 

 vessels are brought to enter the body 

 through the space at which the vitel- 

 lus is shut in to form the primitive gut. 

 There are then two arteries and two veins, 

 which are known as the omphalo-mesen- 

 teric vessels. This form of circulation soon 

 gives way to the placenta!, and the vessels 

 passing to the umbilical vesicle waste, 

 those belonging to that portion of the 

 original vitelline cavity which forms the 

 intestine, becoming the mesenteric vessels. 



What are the prominent features of the 

 placenta! circulation? 



(1) In the arterial circulation some 

 conditions of the heart and great ves- 

 sels are necessary to modify the pulmo- 

 nary circulation before the air enters the 

 lungs at birth. (2) In the circulation of 

 the liver the veins present modifications to allow for the return 

 placental circulation. 



Where do the arteries to the placenta arise? 



They follow the allantois in its growth. Springing from the 

 internal iliac artery of each side, two arteries pass up the umbilical 

 cord and break into the branches in the placenta which terminate 

 in capillary tufts. 



How does the blood return to the body from the placenta? 



By the umbilical vein it is taken to the liver, where part of it cir- 

 culates through the liver-capillaries in the same manner as the blood 

 from the portal vein, the remainder passing through the ductus 

 venosus. 



What is the ductus venosus? 



It is a large vein which appears at the under surface of the 

 liver, and returns a large part of the blood from the umbilical 

 vein directly to the inferior vena cava, without circulation in the 

 capillaries of the organ. 

 How does the ductus venosus appear in adult life? 



Soon after birth the umbilical vein and ductus venosus become 

 an impervious cord extending from the navel to the liver : the 



