In the year 1834 it was known that the ovum of the left the Ornithorhynchus paradoxus left the ovarium with a spherical yelk or vitellus about l½"' (lines) in diameter, and that, having reached the uterine portion of the oviduct, it had acquired a smooth subtransparent chorion or outer tunic separated from the proper membrana vitelli by a clear fluid. Such ova, usually two in number, had been detected in females killed in the month of October, in the left uterus, of sizes ranging from 2½'" to 3½'" (lines) in diameter, without any sign of organization of the chorion, or of preparation for placental adhesion on the uterine wall. The increase of size in the uterine over the ripe ovarian ovum was due to increase of fluid between the chorion and vitelline tunics.