THE EMBRtO 295 



construction, and has principally, if not entirely, a respiratory 

 function. In amphibia the allantois is a mere urinary 

 bladder, and the period of allantoic circulation is not repre- 

 sented. 



As development in the mamma* proceeds, the placenta is 

 separated by a greater distance from the embryo; the original 

 hollow of the urachus and placental part of the allantois 

 disappears, and the placenta is united to the navel by an 

 umbilical cord, consisting of vascular trunks, gelatinous con- 

 nective tissue, and an investment of thickened amnion. The 

 arteries which convey the blood to the placenta are two in 

 number, arising within the pelvis, and pass up one on each 

 side of the urinary bladder. While within the abdomen, 

 they are termed hypogasfoic, and in the cord they are called 

 umbilical arteries. The veins which return the blood from 

 the placenta to the foetus are also at first two in number, 

 but the right one speedily disappears, so that in the cord 

 there are only three vessels, namely, two umbilical arteries 

 and one umbilical vein. These are twisted spirally, the 

 arteries having the appearance of winding round the vein. 

 The umbilical vein, entering at the navel, passes up to the 

 under surface of the liver, where it lies in the longitudinal 

 fissure between the right and left lobes of that organ, com- 

 municates freely with the portal vein, and is prolonged back- 

 wards, under the name of ductus venosus, to join the inferior 

 vena cava. 



216. When it is considered that throughout foetal life the 

 lungs are of no service, and that respiration is carried on by 

 means of the placenta, it will be at once perceived that the 

 course of the circulation must be very different then from 

 what it is afterwards, and that a sudden change must take 

 place at birth. This is really the case; and in connection with 

 the course of the circulation there are some foetal peculiarities 

 of the heart and pulmonary artery. In the fcetal heart the 

 annulus ovalis of the right auricle is an open foramen ovale; 

 and the pulmonary artery, after giving off the right and left 

 pulmonary arteries, which are but small branches, is con- 

 tinued straight on to open into the arch of the aorta, where 

 that artery is about to be continued, as the descending aorta, 

 down through the thorax. This continuation of the pul- 



