The liver forms initially as a diverticulum from the endoderm of the mid-gut around the 25th day of gestation. It develops by dichotomous branching after giving off an unpaired diverticulum, the future gallbladder. Extending into the mesenchyme of the septum transversum, which gives rise to the interstitial tissues and capsule, the irregularly arranged cords of liver cells are mixed with capillary loops, which eventually form sinusoids. The arrangement of definitive liver lobules arises from a series of right-angled branches arising from the primary cell cords, and from these branches a further series of radiating branches develops. These give rise to lobules and the axial cells from which the branches arise differentiate into the branch of the hepatic duct system that drains the lobule.