DEVELOPMENT OF LEPUS 233 



brings carbon dioxide and excretory products which are passed 

 on into the maternal circulation. The placenta therefore 

 functions as a respiratory, nutritive, and excretory organ. At 

 the same time, a certain amount of nutriment is obtained from 

 the glands of the uterus, and is either ingested phagocytically 

 by the trophoblast or absorbed into the blood-vessels of the 

 yolk-sac (the cavity of which opens freely into that of the 

 uterus). But the functions of the placenta do not end there, 

 for it also serves as a store of food material for the developing 

 embryo. In particular, glycogen is accumulated in the 

 placenta at early stages before the embryo has a liver of its 

 own ; when the latter develops, the gycogen content of the 

 placenta decreases. 



The vascular system of the embryo rabbit resembles that 

 of the chick, but the posterior cardinals persist as the azygos 

 and hemiazygos veins. The blood from the placenta arrives 

 in the umbilical veins, of which the right disappears and the 

 left runs into the ductus venosus and so to the right auricle. 

 As in the chick, the septum between the auricles in the heart is 

 perforated, and the oxygenated blood from the placenta can 

 get through to the left auricle, left ventricle, and so to the 

 carotids and brain, which requires the purest blood in the body. 

 The pulmonary artery connects with the aorta on the left side 

 by the ductus arteriosus, so that the remainder of the venous 

 blood in the right auricle passes through the right ventricle, 

 pulmonary artery, and ductus arteriosus to the aorta below the 

 place where the carotids come off, and does not have to go 

 through the lungs. The ductus arteriosus degenerates and 

 the perforation of the interauricular septum is closed at 

 birth when the lungs begin to function. The right systemic 

 arch disappears. 



As in lower forms, the fore gut and the hind gut remain 

 blind for a long time. In these regions the endoderm becomes 

 apposed to the overlying ectoderm forming the oral plate and 

 cloacal plate respectively. Perforation of these give rise to 

 the mouth and cloaca, which latter is divided into anus and 

 urino-genital aperture. The bladder forms from the base 

 of the allantois. 



