AbstractIn an embryo of eleven and one‐half to twelve and one‐half days, the stomach, duodenum, liver and primordium of the dorsal pancreas were exciseden bloc, placed on a rayon grid supported by a grid of stainless steel and cultivated on a liquid medium of cock serum and extract of chick embryo for 8 or 10 or 12 days. The culture was incubated at 37°C in air supplemented by controlled oxygen and carbon dioxide.One‐hundred‐fifteen cultures were fixed in Bouin's solution and the sections stained with aldehyde fuchsin. Forty‐one cultures were fixed in Zenker's solution and the sections stained with hematoxylin and eosin. The 269 control pancreases were of two kinds: beginning controls taken at the time of the explantation (61 specimens); reference controls taken during the period from day eleven and one‐half postcoitum to day five postpartum (208 specimens).From an explant of a pancreatic primordium too primitive to have either islet or acinus, a culture could give rise to islets with granulated beta cells and acini with zymogen granules. The best‐developed islet in a ten‐day culture of an explant from a donor of eleven and one‐half days had an estimated granulation age of twenty‐one and one‐half days; thus the differentiationin vitrohad kept pace with that which occurredin vivoduring equal time in days. The best‐developed acinus in a 12‐day culture of a pancreatic primordium from an embryo of 12 and one‐half days had an estimated developmental age