The fowl’s egg, at about the fortieth hour, shows the buds from which the limbs are developed. A vesicle is seen to protrude near the anal end of the intestine, which, rapidly expanding, spreads over the embryo, acquiring a close adhesion to the amnion, but remaining distinct from the vitellicle, over which it spreads. It finally encloses the albumen and interposes itself between the latter and the lining membrane of the shell. Umbilical vessels are associated with this membrane. Hunter called this membrane the _allantois_ from its containing urine, and Owen states that the sac which surrounds the albumen acts as the chorion or _placenta_; for it is most probable that from this surface the albumen is absorbed and the chick supported on its developmental food. The external part of the sac apparently acts as lungs as it comes into contact with the shell of the egg through pores of which there is an exchange of air. Oxygen is consumed and carbon dioxide is given off. The blood in the vessels of this membrane is in color more like arterial blood and that in the interior more like venous blood. The embryonic mass of the incubating egg always floats to the top side. As the embryo grows it turns upon its left side, exhibiting a profile view; it then indents the yolk, and finally almost divides it into two parts. The peripheral layers of cells rise from the margin of the germ mass, and extend and contract toward the opposite pole. This tract of germ substance is the _primitive streak_. Along the median line it next forms a furrow, which stops short of the ends of the streak. This streak terminates opposite the point from which the germ begins, and swells into the head. The median furrow expands upon it. The cephalic borders are next united by a thin layer of epithelial cells above the furrow, converting it into a cavity, or ventricle. The myelonal furrow is similarly covered by a layer, uniting the lateral columns. The embryonal trace becomes longer, narrower, and bends round the vitellus. A layer of epithelial cells forms a network over the whole dorsal surface of the embryo. Oblique striæ appear in the broadening germ mass radiating from the primitive streak. These indicate divisional segments. These beginnings of aponeurotic septa probably accompany and support nervous productions from the myelon columns. Two transverse constrictions begin to divide the cephalic enlargements into three lobes, the second and the third of which expand into vesicles. An accumulation of cells at the side of the middle expansion appears to add greatly to its breadth. This forms the basis of the eyes.