The heart arises continuously with the sinus venosus, which in the Amniotic Vertebrata is directly continued into the vitelline veins. Though at first it ends blindly in front, it is very soon connected with the foremost aortic arches.
The simple tubular heart, connected as above described, grows more rapidly than the chamber in which it is contained, and is soon doubled upon itself, acquiring in this way an S-shaped curvature, the posterior portion being placed dorsally, and the anterior ventrally. A constriction soon appears between the dorsal and ventral portions.
The dorsal section becomes partially divided off behind from the sinus venosus, and constitutes the relatively thin-walled auricular section of the heart; while the ventral portion, after becoming distinct anteriorly from a portion continued forwards from it to the origin of the branchial arteries, which may be called the truncus arteriosus, acquires very thick spongy muscular walls, and becomes the ventricular division of the heart.
The further changes in the heart are but slight in the case of the Pisces. A pair of simple membranous valves becomes established at the auriculo-ventricular orifice, and further changes take place in the truncus arteriosus. This part becomes divided in Elasmobranchii, Ganoidei, and Dipnoi into a posterior section, called the conus arteriosus, provided with a series of transverse rows of valves, and an anterior section, called the bulbus arteriosus, not provided with valves, and leading into the branchial arteries. In most Teleostei (except Butirinus and a few other forms) the conus arteriosus is all but obliterated, and the anterior row of its valves alone preserved; and the bulbus is very much enlarged[222].
In the Dipnoi important changes in the heart are effected, as compared with other Fishes, by the development of true lungs. Both the auricular and ventricular chamber may be imperfectly divided into two, and in the conus a partial longitudinal septum is developed in connection with a longitudinal row of valves[223].
In Amphibia the heart is in many respects similar to that of the Dipnoi. Its curvature is rather that of a screw than of a simple S. The truncus arteriosus lies to the left, and is continued into the ventricle which lies ventrally and more to the right, and this again into the dorsally placed auricular section.
After the heart has reached the piscine stage, the auricular section (Bombinator) becomes prolonged into a right and left auricular appendage. A septum next grows from the roof of the auricular portion of the heart obliquely backwards and towards the left, and divides it in two chambers; the right one of which remains continuous with the sinus venosus, while the left one is completely shut off from the sinus, though it soon enters into communication with the newly established pulmonary veins. The truncus arteriosus[224] is divided into a posterior conus arteriosus (pylangium) and an anterior bulbus (synangium). The former is provided with a proximal row of valves at its ventricular end, and a distal row at its anterior end near the bulbus. It is also provided with a longitudinal septum, which is no doubt homologous with the septum in the conus arteriosus of the Dipnoi. The bulbus is well developed in many Urodela, but hardly exists in the Anura.
In the Amniota further changes take place in the heart, resulting in the abortion of the distal rows of valves of the conus arteriosus[225], and in the splitting up of the whole truncus arteriosus into three vessels in Reptilia, and two in Birds and Mammals, each opening into the ventricular section of the heart, and provided with a special set of valves at its commencement. In Birds and Mammals the ventricle becomes moreover completely divided into two chambers, each communicating with one of the divisions of the primitive truncus, known in the higher types as the systemic and pulmonary aortæ. The character of the development of the heart in the Amniota will be best understood from a description of what takes place in the Chick.
In Birds the originally straight heart ([fig. 109]) soon becomes doubled up upon itself. The ventricular portion becomes placed on the ventral and right side, while the auricular section is dorsal and to the left. The two parts are separated from each other by a slight constriction known as the canalis auricularis. Anteriorly the ventricular cavity is continued into the truncus, and the venous or auricular portion of the heart is similarly connected behind with the sinus venosus. The auricular appendages grow out from the auricle at a very early period. The general appearance of the heart, as seen from the ventral side on the fourth day, is shewn in [fig. 360]. Although the external divisions of the heart are well marked even before this stage, it is not till the end of the third day that the internal partitions become apparent; and, contrary to what might have been anticipated from the evolution of these parts in the lower types, the ventricular septum is the first to be established.
It commences on the third day as a crescentic ridge or fold springing from the convex or ventral side of the rounded ventricular portion of the heart, and on the fourth day grows rapidly across the ventricular cavity towards the concave or dorsal side. It thus forms an incomplete longitudinal partition, extending from the canalis auricularis to the commencement of the truncus arteriosus, and dividing the twisted ventricular tube into two somewhat curved canals, one more to the left and above, the other to the right and below. These communicate with each other, above the free edge of the partition, along its whole length.