276 



COMPARATIVE ANATOMY. 



definitive urinary bladder and urethra. Indications of the 

 point of exit of the allantois and vitello-intestinal duct (umbilical 

 cord) from the body-cavity can be seen in the adult at the navel, 

 or umbilicus, which represents the last point at which the 

 body-walls become united. 



PC(Chf) 



.Dv 



FIG. 220. DIAGRAMMATIC SECTION THROUGH THE HUMAN GRAVID UTERUS. 



U, uterus ; Tb, Tb, Fallopian tubes ; Uff, uterine cavity ; Dv, decidua vera, which 

 at Pu passes into the uterine portion of the placenta ; Dr, decidua reflexa ; Pf, 

 foetal portion of the placenta (chorion frondosum, Chf) ; Chi, chorion laeve ; A, A, 

 the cavity of the amnion filled with fluid : in the interior of the amnion is seen 

 the embryo suspended by the twisted umbilical cord ; H, heart ; A, aorta ; 

 cs, precaval ; ci, postcaval ; p, portal vein ; Al, allantoic (umbilical) arteries ; 

 f, the liver, perforated by the umbilical -vein ; D, the remains of the yolk-sac 

 (umbilical vesicle). 



The branchial vessels never become functional as such, in any 

 period of development either in Mammalia or Sauropsida, but those 

 which persist give rise to important vascular trunks of the neck, 

 head (carotids), upper extremity (subclavian), and lungs (pulmonary 

 artery), and also to the roots of the aorta, one or both of which may 

 remain (comp. Fig. 221, A to D). 



