7QO 



DEVELOPMENT 



charges which occur at this period. A new mucous membrane is of course 

 gradually developed. 



Circulation of Blood in the Fetus. The circulation of blood in the 

 fetus differs considerably from that of the adult. 



Returning from the placenta by the umbilical vein the blood is first con- 

 veyed to the under surface of the liver, where the stream is divided a part of 

 the blood passing straight on to the inferior vena cava through a venous canal 

 called the ductus venosus, while the remainder passes into the portal vein and 

 reaches the inferior vena cava only after circulating through the liver. It is 

 carried by the vena cava to the right auricle of the heart, into which cavity 



FIG. 505. Diagrammatic View of a Vertical Transverse Section of the Uterus at the 

 Seventh or Eighth Week of Pregnancy, c, c, c', Cavity of uterus, which becomes the 

 cavity of the decidua, opening at c, c, the cornua, into the Fallopian tubes, and at c' into the 

 cavity of the cervix, which is closed by a plug of mucus; dv, decidua vera; dr, decidua 

 reflexa, with the sparser villi embedded in its substance; ds, decidua serotina, involving 

 the more developed chorionic villi of the commencing placenta. The fetus is seen lying 

 in the amniotic sac. The umbilical cord and its vessels pass up from the umbilicus to 

 the distribution of the blood vessels in the villi of the chorion- and the pedicle of the 

 yolk-sac the cavity between the amnion and chorion. (Allen Thomson.) 



the blood is also pouring that has circulated in the head and neck and arms, 

 and has been brought to the auricle by the superior vena cava. It might be 

 naturally expected that the two streams of blood would be mingled in the 

 right auricle, but such is the case only to a slight extent. The blood from the 



