338 OUTLINES OF CHORD ATE DEVELOPMENT 



terior loop of the heart tube, and is now separated from the 

 auricle by the narrowed auriculo-ventricular aperture (Fig. 133). 

 The ventricle occupies the ventral region of the pericardial 



FIG. 133. The development in the heart of the chick. A, F, after Hoch- 

 stetter; B-E , after Greil. A-E . ventral views of the heart; A, of a forty-hour 

 embryo; B, of a 2.1 mm. embryo; C, of a 3.0 mm. embryo; D, of a 5.0 mm. 

 embryo; E, of a 6.5 mm. embryo. F. Frontal section through the heart of a 9 

 mm. embryo, a, Auricle; b, bulbus; d, roots of dorsal aorta; e, median endothelial 

 cushion; t, interventricular groove; la, left auricle; le, lateral endothelial cushion; 

 ft, left ventricle; om, vitelline artery; p, left pulmonary artery; ra, right auricle; 

 rr, right ventricle; s, interventricular septum, so, interauricular septum; t, 

 roots of aortic arches; r, ventricle, 



cavity beneath the auricle. The walls are already thickened 

 and spongy. Anteriorly a slight constriction separates the 



