THE FOETUS AND UTERUS. 101 



(5) "Wharton's jelly, a primitive embryonic tissue composed of 

 branching cells in a mucoid matrix. 



(6) A covering of epiblast. The amnion is attached round the 

 placental insertion of the cord. 



Up to the end of the 4th week the embryo is closely united 

 to the chorion by the short body-stalk (Fig. 75), but in the 

 second month the cord elongates, and in the third month it 

 measures about 12 cm. and about 40 cm. (16 inches) at birth. 



Formation of the Umbilicus. —In the adult the umbilicus 

 marks the point where the umbilical cord was attached. It is 

 the point at which the lateral somatopleuric plates fused and 

 thus shut off the intra-embryonic coelom (peritoneal, pleural and 

 pericardiac cavities) from the extra embryonic coelom which is 

 enclosed between the amnion and chorion (Figs. 70 and 72). If 

 these diagrams be examined the yolk sac will be seen to hang 

 free within the great primitive coelom through the umbilicus 

 which, at this stage, is nearly as extensive as the ventral surface 

 of the embryo (Fig. 75). The embryo during the 3rd week is only 

 from 3 to 5 mm. long (|~£ inch) ; while it grows the umbilicus 

 increases at a slower rate. It thus comes about that when the 

 embryo is an inch (25 mm.) long, the primitive umbilicus, not 

 having kept pace with the body growth, remains comparatively 

 small. The somatic layers gradually contract round the yolk sac 

 and allantoic stalk, the umbilicus and umbilical cord being thus 

 formed. At first the belly end of the umbilical cord is funnel- 

 shaped and a coil of intestine hangs within the umbilicus until 

 the 3rd month of foetal life. After the third month the coil of 

 intestine, probably owing to an increase in the capacity of the 

 abdomen, retreats within the umbilicus, which then contracts 

 round the umbilical vessels. It occasionally happens that the 

 process of development at the umbilicus is arrested at the second 

 month and the child is born with some of its abdominal contents 

 within the umbilicus and upper part of the cord. This condition 

 is known as congenital umbilical hernia. Sometimes, too, the stalk 

 of the yolk sac — the vitello-intestinal canal — persists, giving rise 

 to an umbilical faecal fistula. The cavity of the allantois may 

 also remain open, leading to an umbilical urinary fistula. Fistulae 

 at the umbilicus are, however, comparatively rare. 



