Embryonic hearts were obtained from 78 guinea pig embryos at 20–40 days of gestation. They were frozen quickly, freeze-dried and prepared by the Falck catecholamine fluorescent method for demonstration of adrenergic fibers. Other hearts were fixed in 10% formalin and examined after silver impregnation with the Holmes technique. The results of the two methods were correlated to visualize the total neural pattern as well as the specific adrenergic elements of the developing hearts.
The present study indicates that vagal fibers, accompanied by the primordia of the cardiac ganglia, reach the atrial wall on the twenty-fifth day of gestation in the guinea pig. They penetrate the wall and are distributed by individual branches throughout the atrial wall from days 26 to 29 inclusive. From day 30 to parturition, the basic pattern of atrial distribution is elaborated by the lengthening, thickening, and branching of individual fibers. Sympathetic fibers pass to the atrial wall from the twenty-fifth to twenty-ninth day, those coursing with the vagus nerve arriving on the twenty-fifth day, while the remaining fibers arrive on the twenty-sixth to the twenty-ninth days.
A ventricular ground plexus of sympathetic fibers is present just deep to the epicardium on the twenty-sixth day, and from this point until 30 days of gestation the ground plexus penetrates the ventricular myocardial wall. The sympathetic fibers at first course along the edge of a muscle bundle, but not between muscle fibers. The nerves become thicker at 29 days but do not exceed 2 μ. They branch slightly at 27 days, and at 30 days they are well branched and appear to overlie the surface of the muscle bundles simulating a perimysial plexus. At 40 days a very dense perimysial plexus is visible which contains some fluorescent nerves.
Complete autonomic innervation is established by the thirtieth day of gestation.