INTRODUCTION 



11 



allantois. The latter becomes attached to a definite region of 

 the uterine wall, and from it vascular processes or villi arise, so 

 that the foetal and maternal blood-vessels come into very close 

 relations with one another. Thus an allantoic placenta is 



Pa 



Pf(CLf) 



Dv 



FIG. 9. DIAGRAMMATIC SECTION THROUGH THE HUMAN GRAVID UTERUS. 



A, aorta ; A, 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 ; Al, 

 allantoic (umbilical) arteries ; Chi, chorion laeve ; Z>, the remains of the 

 yolk-sack (umbilical vesicle) ; Dr, decidua reflexa ; Dv, decidua vera, which 

 at Pu passes into the uterine portion of the placenta ; H, heart ; Pf, fcetal 

 portion of the placenta (chorion frondosum, Chf) : Tb, Fallopian tube ; 

 U. uterine wall ; UH, uterine cavity ; ci, posteaval, cs, precaval, and p. 

 portal vein ; f, the liver, perforated by the umbilical vein. 



formed, which serves both for the respiration and nutrition of the 

 foetus (Fig. 9). 



The following important points must now be noted as regards 

 the structure of the Vertebrate body. After the above-mentioned 

 main organs have appeared, a smaller dorsal neural tube and 

 a larger ventral visceral tube are seen to extend longitudinally 

 through the body, and between the two is a rod-like support- 

 ing structure, the notochord (p. 6), which forms the primary 



