3i2 Vertebrate Embryology 



amnion. The side next to the decidua serotina 

 is soft and irregular, and is of a darker, though 

 varying color, because of the blood vessels in 

 it. The villi are separated by furrows into 

 rounded or angular areas of about 25 mm. 

 diameter, the cotyledons. Covering this rough, 

 villous surface and dipping down into the fur- 

 rows just mentioned is a thin membrane, a part 

 of the decidua that clings to the placenta when 

 the latter tears away from the uterus. 



When the child is born the amnion and 

 chorion are ruptured, allowing the amniotic 

 fluid to escape, but the infant remains attached, 

 for a time, to the uterus, by means of the um- 

 bilical cord and the placenta ; then the foetal 

 placenta separates from the maternal and is 

 passed out. This foetal placenta, together with 

 the remains of the amnion and chorion, and 

 portions of the decidua, is known as the after- 

 birth. 



That part of the allantois, in man, which 

 lies in the umbilical cord remains in a rudi- 

 mentary condition, but the intra-embryonic 

 portions undergo further development ; the 

 proximal portion becomes enlarged and hollow 

 to form the urinary bladder, while the part 

 between the apex of the bladder and the um- 



