ABSTRACT
An embryo of 4.3 mm is described. It comes to the laboratory in 10 per cent for-malin as an intact chorion measuring 22 by 16 mm. The embryo is in the shape of theletter C, and its tail is curved. The limb buds are round in shape. There are 4pharyngeal pouches, the 4th being very small. Between the bases of the 1st and 2ndbranchial arches, in the floor of the pharynx, the tuberculum impar has developed. Theotic vesicles are slightly elongated sacs and no longer connected with the covering ecto-derm. The eyes are represented by the optic vesicles and the thickened ectodermal an-lage of the lens. Trachea is still largely a groove in the ventral wall of the pharynx andesophagus. The heart is still in the so-called S shaped stage; the atrium lies on the leftside of the bulb, and has not yet doubled; the sinus venosus is not completely out ofthe septum transversum; in the atrium, near the sinus venosus, the endothelial tube isclosely attached to the wall. There are 4 pairs of aortic arches: the lst pair breaks intocapillaries imbedded in the mandible. The other three join the dorsal aortae which runcaudal and finally unite to form one median dorsal aorta. The anterior cardinal, pos-terior cardinal, common cardinal and vitelline veins, etc. are all paired and symetricallyarranged. The embryo herein described falls in group Ⅷ of Streeter.