Tlie rarity of recorded liumaii twin embryos, especially those with a single cliorioii, has led the author to report this specimen. Although the anatomy and physiology of double monsters and of fetal aiid adult twins have been give11 volumes, aiid complete theoretical explanations of their formation have been proposed, piihlished specimens of twins in embryonic stages to confirm these theories are rare, even in the lower mammals. Corner ('22) collected these and added three pig embryos of his own. Human twin embryos of the earlier moiiths are eveii more rare. Streeter ('19) rcported that in tlie Cariicgie collectioii there were only forty-three specimens, of which all hiit two were over 5 mm. in length. Arey ('22 a, '22 b, '23 t i , '23 b, '23 c), in a comprehensive series of papers, has gathered and analyzed the eases of monochorioiiic twins found in tuba1 pregnancy. Such early emhryos are p a r t i c u l a r l ~ important, siiice they give information in regard to the method lly which twins are presumably derived from a siiigle ovum aiid the time a t which this occiirs. As pointed out by ('oriier, if the embryonic area splits to form twiiis before tlie amnioii is formed, each txviii will lie in a separate amiiioii siirrouiided by a single cliorioii; but if separation occui-s after the amnion is formed, tlie tm-ins ill lie in a commoii amniotic cavity. Of two early specimeiis reported h y h e y , one is of the first type, the other is of the "53