The Dependence of Head Curvature on the Development of the Heart in the Chick Embryo

Abstract

At about the end of the second day of incubation, the anterior end of the embryonic axis of an embryo chick becomes twisted and bent. The twist, which is a simple rotation around the main embryonic axis, brings the left side of the head in contact with the yolk and raises the right side to point vertically upwards. The bending throws the embryonic axis into a rather complicated shape, the essential feature of which is that the anterior tip of the head remains comparatively near to a point just posterior to the ears, while the whole region of neural tube and brain which lies in between these two points is bent into an arc. The posterior part of the arc curves out slightly to the left of the main line of the embryonic axis but much the greater part lies to the right of this line. In the concavity of the curve, on the right side, lies the heart.