VIII.--Such being the phases of metamorphosis of the primary (branchial) arches which yield the vessels in their normal adult condition, we obtain in this history an explanation of the signification not only of such of their anomalies as are on record, but of such also as are potential in the law of development; a few of them will suffice to illustrate the meaning of the whole number:--lst, The interventricular as well as the interauricular septum may be arrested in growth, leaving an aperture in the centre of each; the former condition is natural to the human foetus, the latter to the reptilian class, while both would be abnormal in the human adult. 2nd. The heart may be cleft at its apex in the situation of the interventricular septum--a condition natural to the Dugong, A similar cleavage may divide the base of the heart in the situation of the interauricular septum. 3rd. The partitioning of the bulbus arteriosus may occur in such a manner as to assign to the two aortae a relative position, the reverse of that which they normally occupy--the pulmonary aorta springing from the left ventricle and the systemic aorta arising from the right, and giving off from its arch the primary branches in the usual order. [Footnote 1] 4th. As the two aortae result from a division of the common primary vessel (bulbus arteriosus), an arrest in the growth of the partition would leave them still as one vessel, which (supposing the ventricular septum remained also incomplete) would then arise from a single ventricle. 5th. The ductus arteriosus may remain pervious, and while co-existing with the proper aortic arch, two arches would then appear on the left side. 6th. The systemic normal aortic arch may be obliterated as far up as the innominate branch, and while the ductus arteriosus remains pervious, and leading from the pulmonary artery to the descending part of the aortic arch, this vessel would then present the appearance of a branch ascending from the left side and giving off the brachio-cephalic arteries. The right ventricular artery would then, through the medium of the ductus arteriosus, supply both the lungs and the system. Such a state of the vessels would require (in order that the circulation of a mixed blood might be carried on) that the two ventricles freely communicate. 7th. If the fourth arch of the right side remained pervious opposite the proper aortic arch, there would exist two aortic arches placed symmetrically, one on either side of the vertebral column, and, joining below, would include in their circle the trachea and oesophagus. 8th. If the fifth arch of the right side remained pervious opposite the open ductus arteriosus, both vessels would present a similar arrangement, as two symmetrical ducti arteriosi co-existing with symmetrical aortic arches. 9th. If the vessels appeared co-existing in the two conditions last mentioned, they would represent four aortic arches, two on either side of the vertebral column. 10th. If the fourth