FGETAL MEMBRANES. 117 



pletely dilated, the amnion usually ruptures — "rupture of the membranes" — 

 and all or a part of the amniotic fluid escapes as the "waters." Usually a 

 varying amount of the fluid remains behind the embryo being kept there by the 

 head completely corking the cervix. This escapes with the birth of the child. 

 In some cases the amnion ruptures at the beginning of labor, before there has 

 been any dilatation of the cervix. The dilating must then be done by the 

 child's head or other presenting part. These are much less adapted to the 

 purpose than the bag of membranes and the result is usually a difficult and 

 protracted "dry" labor. Rarely the amnion fails to rupture during labor and 

 the child is born within the intact bag of membranes. Such a child is said to 

 be born with a "caul." 



The Yolk Sac. 



In the human embryo the yolk sac is but a rudiment of the large and im- 

 portant organ found in some of the lower animals. It develops early and at the 

 end of the second week is an almost spherical sac with a wide opening into the 

 intestine (Fig. 121), there being but a slight constriction between the embryo 

 and the yolk sac. During the third week the yolk sac becomes decidedly con- 

 stricted off from the embryo, remaining connected, however, with the intestine 

 by means of a long pedicle, the yolk stalk or vitelline duct (Fig. 123). As the 

 placenta is formed, and at the same time the umbilical cord, the yolk sac becomes 

 incorporated with the former, where it may sometimes be found by careful 

 search after birth, while the yolk stalk becomes reduced to a strand of cells 

 which traverses the entire length of the umbilical cord (p. 134). 



Whatever function the rudimentary human yolk sac has, must be performed 

 early, as both sac and stalk soon undergo regressive changes. Although no true 

 yolk is present, the sac at first contains fluid and its thick outer mesodermal layer 

 is the place of earliest blood and blood vessel formation. This would seem to 

 indicate that like the larger yolk sac of lower animals, the human yolk sac 

 serves temporarily as a blood-forming organ. 



In about three per cent, of cases that portion of the yolk stalk which lies 

 between the intestine and the umbilicus fails to degenerate, retaining its lumen 

 and its connection with the intestine. It is then known as Meckel's diverticulum 

 and is of considerable surgical importance, as it may become invaginated into 

 the small intestine and thus cause obstruction of the bowel. The blind end of 

 the diverticulum may remain attached to the umbilicus, or it may become free, 

 or in rare cases the stalk may retain a lumen from the intestine to the umbilicus, 

 through which faces may escape— " faecal fistula." Occasionally a portion of 

 the gut from which the yolk stalk i's given off extends for a short distance into 

 the cord. If, as is sometimes the case, this extension fails to retract before 

 birth, a congenital umbilical hernia is the result (see Chap. XIX). 



