Section 30. the Heart in the Fowl Appears First As a Pair of Vessels,

which unite to form a straight trunk in the median line, as the flattened-out embryo closes in from the yolk. The way in which this straight trunk is thrown, first of all, into the S shape of the fish heart, and then gradually assumes the adult form, is indicated roughly by Figure 3. In one respect the development of the heart does not follow the lines one would expect. Since, between the fish and the higher form comes the condition of such an animal as the frog, in which the auricles are divided, while there is only one ventricle, we might expect a stage in which the developing chick's heart would have one ventricle, and a septum between the auricles. But, as a matter of fact, the ventricles in fowl and rabbit are separated first, and the separation of the auricles follows, and is barely complete at birth.