CHORION 



351 



same number of allantoic veins. The allantois also receives 

 the fetal urine. 



As the true placental circulation is established and the vis- 

 ceral plates close the abdominal cavity, the allantois is con- 

 stricted at the umbilicus so as to be divided into two parts. 



FIG. 108. Diagrammatic sections of embryo. 



Showing the destiny of the yolk-sac, ys. vt, vitelline membrane; pp, pleuro- 

 peritoneal cavity; ac, cavity of the amnion; a, amnion; a', alimentary canal; 

 ys, yolk-sac. 



That outside the body shrivels and is cut away with the um- 

 bilical cord at birth, while that inside the body becomes the 

 first part of the male and the whole of the female urethra, 

 the bladder and the urachus. 



Chorion. The chorion is the outer surrounding mem- 

 brane of the embryo after the appearance of the amnion. It 

 consists of three layers. From without inward these are the 



