270 EMBRYOLOGY. 



I shall give a detailed description of its finer structure for the 

 end of pregnancy only, and shall consider especially the following 

 parts: (1) the gelatin of WHARTON, (2) the umbilical vessels, (3) 

 the remnant of the allantois, of the vitelline duct, and of the vasa 

 omphalomesenterica, (4) the amniotic sheath. 



(1) The gelatin of WHARTON forms the common matrix in which 

 the remaining parts are imbedded. It is a gelatinous or mucous tis- 

 sue. In this soft gelatinous substance there run strands of connective- 

 tissue fibrillaB and elastic fibres, which are the scantier the younger 

 the umbilical cord. They are joined together into a network, the 

 meshes of which are narrower at some places than at others. In 

 this way there are formed in the gelatin numerous firm peculiarly 

 differentiated strands. The cells of the gelatinous connective tissue 

 are partly spindle-shaped, partly stellate elements, the latter with 

 widely branching processes. 



(2) The umbilical blood-vessels consist of two large arteries (art. 

 umbilicales), which conduct the blood from the embryo to the pla- 

 centa, and a capacious vena umbilicalis, in which the blood flows back 

 to the embryo after having traversed the placental circulation. The 

 two arteries are wound spirally, like the umbilical cord itself, and 

 are joined to each other by an anastomosis near their entrance 

 into the placenta. They are very contractile, and exhibit a thick 

 muscular membrane (tunica muscularis), consisting of circular and 

 longitudinal fibres. 



(3) The canal of the allantois and the vitelline duct, which are 

 essential components of the umbilical cord during the first months of 

 pregnancy, subsequently undergo reduction, and are present at the 

 ^nd of embryonic life only in the form of insignificant remnants, 

 as has been shown by KOLLIKER, AHLFELD, and HUGE. The canals 

 lose their lumens; there then exist in the gelatin of WHARTON solid 

 cords of epithelial cells ; finally, these also disappear in part, so that 

 only here and there strands and nests of epithelial cells have been 

 preserved. The vitelline blood-vessels (vasa omphalomesenterica), 

 which have a r61e to perform at the beginning of development, soon 

 become inconsiderable, and diminish more and more in comparison 

 with the enlarging umbilical blood-vessels. In the mature umbilical 

 cord they are very rarely to be demonstrated (AHLFELD) ; usually 

 they have wholly degenerated. 



(4) At the beginning of development the amnion forms around 

 the allantoic canal and the vitelline duct a sheath, which can be 

 removed. Afterwards the sheath is firmly fused with the gelatin 



