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Abstract

Bloodstream flow patterns have been outlined in the arterial outflow tract (ventricular outflow tract and bulbus arteriosus) of the chick embryo heart during the period in which septation takes place, Hemodynamic factors underlying flow changes during this period are discussed.

The mapping of flow patterns did not support the concept of a conoventricular flange reported previously. Septation was found to take place between two separate and discrete bloodstreams.

The cellular nature of the aorticopulmonary septum has been described. The spiral ridges that form this septum expand by cellular growth, explaining the ability of this septum to develop against the direction of blood flow. The aorticopulmonary septum divides about two-thirds of the arterial outflow tract; the final partitioning of the most proximal portion of the outflow tract was found to take place by means of the apposition of endocardial cushion tissue masses.

Failure of aorticopulmonary septum development (truncus arteriosus communis persistens) was found to follow fusion of the bloodstreams in experimental studies. In experimental aortic stenosis the appearance of a small left stream was found to be followed by the development of a stenotic aorta. Thus in the first instance the septum apparently cannot develop unless the streams remain separate and in the second case the size of the prhnordial bloodstreams appears to determine the diameter of the vessel.