AbstractThe effects of theAygene on the normal development were investigated by using a inbred strain of C57BL/6 (a/a) and its congenic strain of C57BL/6‐Ay(Ay/a) mice. Three mating groups (female× male), i. e., group I,a/a × a/a; group II,a/a × Ay/a; and group III,Ay/a × a/a, were set, and the rating of normal development was compared among the groups on gestation days 13, 14, 15 or 16 with special attention to the secondary palate. On day 13, the palates were wide‐open and the palatal shelves were vertical in all embryos in all groups. In group I, both shelves became horizontal in 63.3% of the embryos on day 14. The incidences of embryos having completely closed palates were 95.8% and 100% on days 15 and 16, respectively. In group II, in which half of the embryos were expected to carry theAygene, the frequencies of shelf horizontalization on day 14 (31.1%) and of the complete closure on day 15 (74.3%) were significantly lowered as compared with those in group I, although the external morphological rating was comparable. On day 16, however, the palatal closure was completed in almost all embryos. In group III, in which maternal mice as well as half of their embryos carried theAygene, the external morphological rating was delayed as compared with that in either group I or II, and the frequencies of shelf horizontalization on day 14 and of the complete closure on day 15 were as low as 10.9% and 39.6%, respectively. The palatal closure was not yet completed in 15.3% of the embryos on day 16. These results indicate that theAygene causes a delay in shelf horizontalization and closure of the secondary palate of mouse embryos as compared with the