PLACENTA 



669 



thus formed is the placenta, and from it outgrowths penetrate 

 into the uterine wall, expanding the original villi of the tropho- 

 blast and obtaining nourishment and exchanging gases with 

 the maternal blood in lacunae which are formed around them 

 by the breakdown of blood vessels in the wall of the uterus. 

 Thus, as in the chick, the blood in the allantoic vein is arterial, 



mm 



umh 





Fig. 524. — Transverse section of the uterus of a rabbit with an embryo of the 

 nineteenth day. The placenta and membranes are now attached only by a 

 narrow stalk to the embryo, which lies on its side. — Partly after Marshall. 



pi., Placenta ; umb., umbilical stalk ; x, dotted line indicating position of vanished ventral wall of yolk 



sac. Other letters as in Fig. 522, 523. 



though it has here also an important function as the vehicle of 

 nourishment during the greater part of embryonic life. The 

 arrangement, resembling that in the chick (p. 66i), by which 

 this arterial blood is directed into the left auricle, leaves traces 

 in the heart of the adult (Fig. 354 and its legend). The navel 

 of the adult marks the site of the umbilical cord, in which the 

 stalks and blood vessels of the yolk sac and allantois entered the 

 body. The urinary bladder is formed from a remnant of the 



