In the amphibia the heart is a tube, so twisted on itself that the original posterior end is carried forward to the anterior end, and this part, the auricle, is divided lengthwise by a partition into a right and a left side. In the reptiles the ventricle is also partially separated into two chambers, completely so in the crocodiles. In birds and mammals the auricular and ventricular septa are complete in the adult, and the ventral aorta that carries the blood forward from the heart is completely divided into two vessels, one of which now carries blood to the lungs. When we examine the development of the heart of a mammal, or of a bird, we find something like a parallel series of stages, apparently resembling conditions found in the different groups just described. The heart is, at first, a straight tube, it then bends on itself, and a constriction separates the auricular part from the ventricular, and another the ventricular from the ventral aorta. Vertical longitudinal partitions then arise, one of which separates the auricle into two parts, and another the ventricle into two parts, and a third divides the primitive aorta into two parts. In the early stages all the blood passes from the single ventral aorta through the gill-arches to the dorsal side, and it is only after the appearance of the lung-system that the gill-system is largely obliterated. We find here, then, a sort of parallel, provided we do not inquire too particularly into details. This comparison may be justified, at least so far that the circulation is at first through the arches and is later partially replaced by the double circulation, the systemic and the pulmonary. A few other cases may also be added. The proverbial absence of teeth in birds applies only to the adult condition, for, as first shown by Geoffroy Saint-Hilaire, four thickenings, or ridges, develop in the mouth of the embryo; two in the upper, two in the lower, jaw. These ridges appear to correspond to those of reptiles and mammals, from which the teeth develop. It may be said, therefore, that the rudiments of teeth appear in the embryo of the bird. This might be interpreted to mean that the embryo repeats the ancestral reptilian stage, or, perhaps, the ancestral avian stage that had teeth in the beak; but since only the beginnings of teeth appear, and not the fully formed structures, this interpretation would clearly overshoot the mark. The embryo of the baleen whale has teeth that do not break through the gums and are later absorbed. Since the ancestors of this whale probably had teeth, as have other whales at the present time, the appearance of teeth in the embryo has been interpreted as a repetition of the original condition. Some of the ant-eaters are also toothless, but teeth appear in the embryo and are lost later. In the ruminants that lack teeth in the front part of the upper jaw, _e.g._ the cow and the sheep, teeth develop in the embryo which are subsequently lost.