600 



THE HUMAN EMBRYO. 



along which the allantoic vessels can pass from the embryo to 

 the placenta (Fig. 198, aa, va) ; and the explanation of the pre- 



Fig. 254.— A diagrammatic section of the pregnant Human Uterus at the 

 seventh or eighth week. (From Quain's ' Anatomy,' after Allen Thomson.) 



al, allantoic stalk, am, true amnion ; the part shaded horizontally, between the 

 amnion and the embryo, is the amnionic cavity, c, cavity of the uterus, c', plug of 

 mucus in the cervix uteri, ch, chorion, dr, decidua reflexa. ds, decidua serotina. r/i>, 

 decidua vera, i, intestine of embryo, u, allantoic arteries, y, yolk-sac. y', yolk-stalk. 



cocious appearance of the allantoic stalk, and probably also of 

 the peculiarities of its development, in human embryos, is to 

 be found in the importance of establishing vascular relations 



