Pregnant ddN mice were injected intraperitoneally with 15,000 IU of water-miscible vitamin A on the eighth day of pregnancy, and fetuses were removed from the mice on the eighteenth day. Pregnant Wistar rats were injected intraperitoneally with 200,000 IU per kg body weight of water-miscible vitamin A on the tenth day of pregnancy, and fetuses were removed from the rats on the twenty-first day. The pathological changes of the malformed ears of the mouse and rat fetuses were observed by stereoscopic microscopy after combined staining of bone and cartilage. The incus, stapes and Reichert's cartilage all were malformed or absent in most of the mouse ears, much more than in the rat ears. The malleus and Meckel's cartilage were normally shaped in most of the ears of both animals. The author concluded that a considerable part of the incus is derived or originated from the second arch cartilage and that the malleus is derived entirely from the first arch cartilage.