The detailed study of the later development of the blood system shows that it provides one of the most interesting chapters in vertebrate embryology. The fundamental plan is seen to be that of two main longitudinal blood vessels, one ventral to the ali mentary canal, in which the blood runs forwards, and one dorsal (dorsal aorta), in which the blood streams in a tailward direction, these two longitudinal vessels being connected by a series of hoop like aortic arches, situated between the gill-clefts, in which the blood passes from the ventral vessel to the dorsal. The vertebrate heart consists of the portion of the ventral vessel immediately behind the pharynx, in which contraction of the vessel wall, else where comparatively inconspicuous, becomes greatly exaggerated and occurs rhythmically throughout life.
The heart or cardiac tube is that part of the ventral vessel which is contained within the pericardiac chamber. At first straight, its rapid increase in length, combined with the fact that it is fixed at each end where it traverses the pericardiac wall, causes it to assume a characteristic S-shaped curvature. As may be well seen in the embryo of a f owl during the third day of incubation, waves of contraction pass forwards along the cardiac tube, propelling the blood in its interior forwards towards the aortic arches. As development proceeds the originally uniform cardiac tube becomes at intervals relatively enlarged to form a series of four dilatations, demarcated from one another by rela tively less dilated portions. These four dilatations become the sinus venosus, atrium, ventricle and conus arteriosus. With this morphological change in diameter comes a physiological change in that the originally uniform wave of contraction becomes replaced by serial contractions of each chamber in turn. As development proceeds further, the four chambers become compacted together and the original tubular shape of the heart is completely lost. The pumping activity of the heart becoming more and more con centrated in the ventricle, the muscular wall of this part becomes much more highly developed than that of the others.
To secure that the blood stream flows in the proper direction a valvular apparatus becomes developed in the interior of the heart and this, in its earliest stages, takes the form of longitudinal ridge-like thickenings of the inner layer of the heart-wall. These are best seen in the conus arteriosus, where they are normally four in number and where they are jammed together when the conus contracts, obliterating its cavity and so preventing any backward suction when the ventricle dilates. These endocardiac
ridges, dependent for their efficiency upon physiological activity, become in such relatively archaic vertebrates as elasmobranchs and ganoids converted into a purely mechanical apparatus which works automatically, each ridge becoming segmented into a row of valves shaped like watch-pockets, with their openings directed towards the head. These flatten against the wall when the blood streams forwards, but open out and occlude the cavity by their edges coming in contact the moment the blood tends to re gurgitate. In the air-breathing vertebrates from lungfish upwards, the conus with its valvular apparatus undergoes an extraordinarily interesting series of evolutionary changes. In the lungfish the conus is relatively long and is bent into a characteristic Z-form. Along its interior run the four endocardiac ridges. Two of these, the right and the left, project as thin blade-like structures more than halfway across the lumen, their free edges overlapping so that they subdivide the cavity into two portions, one dorsal and one ventral. The two cavities are continued forwards into the ventral aorta by a horizontal partition which extends as far for wards as the level of aortic arch V. where it merges into the roof of the ventral aorta Aortic arches V. and VI. take origin from the dorsal or pulmonary cavity, while the remaining aortic arches spring from the continuation forwards of the ventral cavity. The atrio-ventricular portion of the heart also has its cavity divided, in this case by a vertical septum projecting forwards from its posterior wall. This septum is incomplete, not extending com pletely across the atrio-ventricular cavity except when the wall of this part of the heart is contracted. Owing to the peculiar flexure of the conus, its incomplete septum—horizontal at its front end—becomes at its ventricular end vertical and in line with the atrio-ventricular septum. The result is that, in the contracted condition of the ventricle, the right half of its cavity is continu ous with the right half of the cavity of the conus and this cavity, owing to the peculiar flexure, ends off at its headward end by being dorsal, i.e. continuous with the pulmonary cavity of the ventral aorta. Correspondingly the left ventricular cavity is con tinued through the conus to the ventral or systemic cavity of the ventral aorta.