364 Veterinary Obstetrics 



In carnivora, the placentae are zonular, the chorionic villi 

 are restricted to a zone encircling the embryonic sac and the an- 

 terior and posterior poles of the chorion are devoid of villi. 



The Umbilical Cord. 



The umbilical cord .serves chiefly as a bond of communication 

 between the embryo and the placenta. It is formed by the allan- 

 toic stalk, surrounded by the amnion, and includes the vestige or 

 remnant of the vitelline duct. In the mare and carnivora it may 

 be divided into the amniotic and allantoic portions. The am- 

 niotic portion, or the umbilical cord proper, extends from the 

 umbilicus of the embryo, through the amniotic cavity, to open 

 into the cavity of the allantois. The allantoic section of the 

 umbilic cord extends across the allantoic sac, from the amnion 

 to the allantois-chorion. Cro.ss-section of the umbilical cord in 

 the amniotic portion reveals the following .structures : the am- 

 nion, serving as a thin, dense, investing membrane ; the two 

 umbilical arteries, the pair of umbilical veins, .sometimes fused 

 to constitute a single vessel ; the urachus and the ve.stiges of 

 the vitelline duct. Around the.se ves.sels, filling out the amni- 

 otic sheath and investing the urachus and vessels within to con- 

 stitute a more or less even cord, is a gelatinous substance, the 

 Whartonian gelatin, embedded in a network of delicate connec- 

 tive tissue. 



The allantoic portion of the umbilic cord consists essentially of 

 the ma.ss of umbilic vessels, which have now become more or less 

 branched. Partially surrounded by, and adherent to, the.se vessels 

 may exist more or less evident remnants of the vitelline duct 

 and sac, prominent in the very early stages of fetal life, rapidly 

 disappearing more or less completely at a later date. The urachus 

 ends in a funnel-shaped dilation just beyond the amniotic sac, to 

 become continuous with the allantoic sac. 



During the earlier stages of gestation there normally projects 

 into the umbilical cord one or more loops of intestine and, in 

 some cases, portions of the liver or of other viscera. The pro- 

 truding intestinal loop escapes through the umbilic ring of the 

 .somatopleure, beside the allantoic stalk, inside the skin and 

 amniotic sheath. The protrusion through the umbilicus ap- 

 parently occurs becau.se the abdominal cavity has not developed 



