THE UMBILICAL VESICLE. 



43 



between the umbilical sac and the embryo has become reduced to a contracted channel 

 extending from the now rapidly closing ventral body-wall to the yolk-sac, which is 

 still, however, of considerable size. The succeeding fifth (Fig. 50) and sixth weeks 

 effect marked changes in the umbilical duct, now reduced to a narrow tube, which 

 extends from the embryo to the chorion, where it ends in the greatly diminished 

 vitelline sac. The lumen of the umbilical duct is conspicuous during the earliest 

 months of gestation, but later disappears, the entoblastic epithelial lining remaining 

 for a considerable time within the umbilical cord to mark the position of the former 

 canal. 



The chief factor in producing the elongation of the umbilical duct is the rapid 

 expansion of the amnion ; with the increase in the amniotic sac the distance between 

 this envelope and the embryo increases, until the amnion fills the entire space within 



Fig. 50. 



^., 



C 



Amnion 



Umbilical \  



vesicle . ' 

 (yolk-sac) 



Tnner surface 

 of chorion 



Chorionic villi 

 of outer surface 



Chorionic sac of thirty-five day embryo laid open, showing embrj'o enclosed b> amnion. X2. 



the chorion, against which it finally lies. In consequence of this expansion, the 

 attachment between the embryo and the amnion around the ventral opening, which 

 later corresponds to the umbilicus, becomes greatly elongated and narrowed. At 

 this point the tissues of the embryonic body-wall and the amniotic layers are directly 

 continuous. The tubular sheath of amnion thus formed encloses the tissue and 

 structures which extend between the embryo and the chorion, as the constituents of 

 the belly-stalk, together with the umbilical duct and the remains of the vitelline 

 blood-vessels ; the delicate mesoblastic layer of the amnion fuses with the similar 

 tissue of the allantois, the whole elongated pedicle constituting the umbilical cord or 

 funiculus. The latter originates, therefore, from the fusion of three chief com- 

 ponents, the amniotic sheath, the belly-stalk, and the vitelline duct ; the belly-stalk. 



