134 TEXT-BOOK OF EMBRYOLOGY. 



cause of this twisting is not known. In places where the turns are quite abrupt 

 and there are considerable accumulations of connective tissue, the cord has a 

 knotted appearance. These points are known as false knots. Rarely the cord 

 is actually tied into a more or less complex knot — true knot — probably due to 

 movements of the foetus. 



4. Remnants of the allantoic stalk and of the yolk stalk. These, if present, 

 are continuous or broken cords of epithelial cells. Rarely one or the other may 

 retain its lumen or some of the yolk stalk vessels may remain. 



As the yolk stalk is carried around to be incorporated as part of the umbilical 

 cord there is enclosed with it a small part of the extraembryonic body cavity. 



The human umbilical cord averages 50 cm. in length and has a diameter of 

 about 1.5 cm. 



The Expulsion of the Placenta and Membranes. — After the birth of the 

 child, the uterine contractions usually cease temporarily and the uterine walls 

 remain contracted around the placenta. In the course of a few moments the 

 uterine contractions are resumed and the placenta and membranes are ex- 

 pelled as the after-birth. 



The line of separation of the placenta and of the decidua parietalis from the 

 uterine mucosa is through the deeper part of the spongy layer (Fig. 113). By 

 this separation many blood vessels are opened, the hemorrhage being con- 

 trolled by the firm contractions of the uterine muscle. The condition of the 

 uterine mucosa, after child-birth, has been described as an exaggeration of its 

 condition at the end of menstruation. Reconstruction of the mucosa takes 

 place by proliferation of the still remaining connective tissue and of the gland- 

 ular elements. 



Anomalies. 



The manner in which the placenta is formed — by excessive development of 

 the decidua and chorion over a limited area and atrophy of the chorion through- 

 out the remainder of its extent — suggests the most frequent variations from the 

 normal. 



The villi instead of developing over the usual discoidal area may develop along 

 a band-like area which more or less completely encircles the chorion. This gives 

 rise to an annular placenta similar to that seen in the Carnivora. Continued 

 development of the villi over the entire chorion may occur. This results in a 

 thin "placenta membranacea." Such a placenta is apt to be adherent and may 

 thus cause a serious postpartum condition. Failure of the villi to atrophy and 

 their continued development over more than a single area give rise to variations 

 in form and number of placentas. When there are two not very distinctly 

 separated areas the condition is known as placenta bipartita. Two completely 

 separated placenta? with distinct branchings of the umbilical vessels to supply 



