The development of the heart in human embryos

Abstract

1. As a function of myocardial cells the rythmic contractions of the myocardium and their contraction in contractile waves progress after Streeter from the sinus region through the atrium, ventricle, and bulb to the aortic sac very early perhaps in 14 somites embryos.
2. In these early periods has the heart tube a gelatinous reticulum layer on the inner surface, which is lacking on the spots where the ventricular pouches begin to develope.
3. As the development of the ventricular pouches is also very early perhaps in ca 20 somites embryos, there occur slowly but steadily the pulmonary and aortic currents. In our XIV developmental stage embryo the pulmonary and aortic parts in the aortic trunk can be distinguished.
4. In our XV stage 6mm long embryo there protrude ventrosinistral and dorsodextral distal bulbar ridges, so that the pulmonary and aortic parts are partly divided. The proximal bulbar ridges are yet obscure in this embryo.
5. In our XVI stage 7.5mm long embryo the pulmonalis and aorta are perfectly separate in the distal part, where the positions of the semilunar valves are certain. But there remains a spacious common room for both in the proximal part of the bulb. There is slight continuation of the reticular tissue of the bulbar ridge to the atrioventricular cushion.
6. The right and left atrioventricular ostium are separated completely in our 7.5mm long one, by which the cranial and caudal cushions contact with each other except both ends of them. The free edge of the interventricular septum is fused at its right side with the right part of the caudal atrioventricular cushion.
7. This position of the fusion of the caudal cushion and interventricular septum indicates that it locates direct left of the right atrioventricular ostium. As for the right atrioventricular ostium except two tubercles of the two atrioventricular cushions there is a lateral cushion which is the proliferation of the right part of the bulbar septum.
8. The free edge of the bulbar septum, which almost completely separates the pulmonary from the aortic part, fuses with its left fore part with the anterior and right side of the free edge of the interventricular septum, the remaining part is protruding to the space between the aortic part and the right atrioventricular ostium. So this free edge surrounds with the two tubercles of the right ends of the cranial and caudal atrioventricular cushions the communication between these two parts.
9. Such surrounding is very narrrow in our 14 mm long one, which is in the later stage of the Streeter's horizon XVII. In this specimen the free edge of the bulbar septum is very near to the right end of the caudal atrioventricular cushion. The caudal cushion is in close connection with the cranial, so that two right tubercles are in the near.(Fig. 19-24, Pl. 4)
10. These neighbouring parts of the reticular cushions; the free ends of the bulbar septum, two right tubercles of the atrioventricular cushions are to close completely the right and left heart. The last continuation of the right and left is observable in our 18mm (a) long embryo (Fig. 25, 26, Pl. 5).
11. The complete separation of the heart occurs in the Streeter's horizon XVIII developmental stage, in which period three semicircular canals of the membranous labyrinth are complete.
12. The conducting atrioventricular bundle and annulus fibrosus are discernible already in the horizon XVI stage embryo, in the next depelopmental stage XVII they are distinct, and except these there is also the atrioventricular node in this 10mm long embryo of XVII stage.