Through the cooperation of the Yale Laboratories of Primate Biology and the Carnegie Laboratory of Embryology, an implanted ovum of the chimpanzee, slightly younger than the earliest human ovum, has been secured. The circumstances leading to this achievement and the similarity of this youngest anthropoid to the human embryo render the occasion of interest to the gynecologist, the embryologist and the anthropologist alike.
The success that attended the enterprise was no mere accident, though a certain amount of good fortune favored us. The time of ovulation was calculated from data collected by the staff of the Yale station over a period of years. At the suggestion of Dr. Robert M. Yerkes, director of the Yale Laboratories, one of us (J. H. E.) set out to determine the relation of ovulation to externally manifest events of the menstrual cycle, namely to the menstrual flow and, more precisely, to the intermenstrual swelling
ELDER JH, HARTMAN CG, HEUSER CH. A TEN AND ONE-HALF DAY CHIMPANZEE EMBRYO, "YERKES A". JAMA. 1938;111(13):1156-1159. doi:10.1001/jama.1938.02790390012004