Printed in Great Britain The dependence of duodenal differentiation in chick embryos on pars distalis hormones

Duodenal differentiation in normal chick embryos between 15-75 and 20-5 days of incuba-tion was characterized by the following changes: 1. The dry weight increases from 4-1 to 12-4 mg. 2. The alkaline phosphatase activity increases from less than 12 to 426 units. 3. The length of the villi increases sixfold. 4. The height of the epithelial cells at the villous tips increases from 12-9 to 25-9 fi. 5. The shape of the mucosal cells changes from low columnar, to cuboidal, to high columnar. 6. The shape of the nuclei progresses from round to ovoid. 7. At first mitotic figures are distributed throughout the epithelium but become restricted to the crypts of Lieberkiihn. 8. Cytoplasmic glycogen appears by day 15-75 and is mobilized by 20-5 days. 9. A mucopolysaccharide at the brush border of the mucosal cells progressively appears at 16-75 days and increases in amount. 10. Alkaline phosphatase activity (Gomori technique) at the brush border appears in low levels at 16-75 days and becomes more intense. 11. Fresh body weights and third toe lengths at 19-75 and 20-5 days of incubation were recorded as indices of body growth. In 'hypophysectomized ' embryos at 19-75-21-5 days of age: 12. The level of duodenal differentiation approximated that of normal.16-75—17-75 day embryos. 13. In 'hypophysectomized ' embryos which received a pars distalis chorioallantoic homo-graft at 9-5 days of incubation the duodena were normal. 14. In 'hypophysectomized ' embryos with grafts which became atrophic the level of duo-denal differentiation was not different from that of untreated 'hypophysectomized ' embryos. 15. In chick embryos duodenal differentiation depends on pars distalis hormones