FCETAL MEMBRANES. 119 



such elimination is carried on through the placenta and there is consequently 

 no need for the development of a large allantoic sac. 



With development of the placenta, that part of the allantoic stalk which lies 

 in the umbilical cord atrophies. Of the embryonic portion of the allantois, 

 or the urachus, on the other hand, the proximal end communicates with the 

 urinary bladder, while the remainder, which extends from the bladder to the 

 umbilicus becomes transformed into a fibrous cord, — the middle umbilical 

 ligament (page 404). Rarely that portion of the allantoic stalk between the 

 bladder and the umbilicus remains patent and opening upon the surface 

 forms a " urinary fistula," allowing urine to escape. 



In Reptiles and Birds the omphalomesenteric vessels, passing along the yolk 

 stalk and ramifying in the mesodermal layer of the yolk sac, convey the nutrient 

 materials of the yolk to the growing embryo. Since the allantois is an organ of 

 respiration and excretion, the allantoic or umbilical vessels have nothing to do 

 with the actual nourishment of the embryo (p. 242). In Mammals the yolk sac 

 is of less functional value. Consequently the vitelline vessels, although present 

 (Fig. 215), play a less important r61e in conveying nutriment. The allantoic 

 (umbilical) vessels, instead of ramifying in the wall of the allantois, as in the 

 lower forms, come into connection with the chorion, passing primarily through 

 the belly stalk. Since the chorion becomes the organ of interchange between 

 the embryo and the mother, the allantoic vessels assume a new function, the 

 allantoic (umbilical) vein carrying food material from the mother to the em- 

 bryo, the arteries carrying waste products from the embryo to the mother. 

 Thus in Mammals, as the yolk sac and vitelline vessels come to play a less im- 

 portant r61e in the nutrition of the embryo, the allantoic vessels, in connection 

 with the chorion, become practically the only means by which the embryo 

 receives its food-supply. 



The Chorion and the Decidua. 



When the fertilized ovum reaches the uterus it becomes fixed or embedded 

 in the uterine mucosa. Fixation usually occurs in the upper half of the uterus 

 but may occur near the cervix. Rarely the ovum becomes fixed to the mucous 

 membrane of the tube instead of to that of the uterus, and, developing there, 

 gives rise to a "tubal" pregnancy— one of the forms of extrauterine gestation. 



Until recently, it was believed that the ovum became attached to the surface 

 of the mucous membrane. Recent studies upon some of the youngest human 

 ova and upon those of some of the lower Mammals, however, seem to indicate- 

 that the ovum in some way pushes itself into— buries itself— in the uterine 

 mucosa (Fig. 105). It is argued that if the ovum simply attaches itself to the 

 surface of the mucosa, one would expect to find, for a time at least, epithelium 

 between the attached surface and the stroma. In a very young human ovum 



