THE F(ETAL MEMBRANES OF MAN. 269 



about as thick as the little finger (11-13 mm. or 0*5 inch), and attains 

 the considerable length of 50 to 60 cm. (20-24 inches). It almost 

 always exhibits a very pronounced spiral twist, which, regarded from 

 the embryo, runs usually from left to right. 



There are often knot-like thickenings of the umbilical cord, which 

 may be due to either of two causes. For the most part they are 

 due to an increased growth here and there in the connective -tissue 

 matrix of the cord (false knots). More rarely they are formed by 

 a knotting of the cord, which results from the fact that the embryo, 

 in the motions which it executes in the amniotic fluid, accidentally 

 slips through a loop of the cord and then gradually tightens it into 

 a knot. The thickening then presents, in distinction from the other, 

 a true knot. 



The attachment of the umbilical cord to the placenta ordinarily 

 takes place in or near its middle (insertio centralis). However, 

 exceptions to the rule are not rare. Thus one distinguishes in addi- 

 tion an insertio marginalis and an insertio velamentosa. In the first 

 case the umbilical cord unites with the margin of the placenta ; in 

 the second place it does not reach the placenta at all, but attaches 

 itself at a lesser or greater distance from the margin of the latter, 

 to the foetal membranes themselves, and sends out from that point 

 the outspreading large branches of its vessels to the placenta. 



Man is distinguished from almost all of the remaining Mammals 

 by the possession of a long slender umbilical cord. Its condition 

 in Man results from the great distension of the amniotic sac. 

 Whereas this at first lies close upon the body of the embryo, it sub- 

 sequently becomes so distended (compare fig. 144 with fig. 143) that 

 it fills the whole cavity of the blastodermic vesicle and everywhere 

 clings closely to the inner surface of the chorion. Owing to this, 

 the remaining structures the yolk-sac with its blood-vessels, the 

 slender canal of the allantois with its connective- tissue envelope, and 

 the umbilical blood-vessels which emerge through the dermal navel 

 of the embryo into the extra- embryonic body-cavity and betake 

 themselves to the chorion, become more and more hemmed in by 

 the amnion, and finally are crowded together into a small cord. 



At first the umbilical cord is short, since it pursues a straight course 

 in uniting the navel of the embryo to the foetal membranes ; after- 

 wards it becomes greatly elongated and folded in the amniotic fluid. 



Its structure varies at different times during pregnancy corre- 

 sponding to the changes which the yolk-sac and the allantois with 

 their blood-vessels undergo. 



