The effect of incubator temperature on mortality rates and on appearance of congenital malformations in white leghorn chicks and embryos was studied. The elevation of incubation temperature to 39.44°C led to a high mortality and to malformations in the eyes and the nervous system, as well as to abnormal torsion of the body axis. It did not produce any cardiac abnormalities, which would have been detected easily because the mortality peak occurred after the development of the heart was complete. Decreasing temperature of incubation to 35.83°C produced a high mortality rate and, very frequently, interventricular septal defects. These results and the possible mechanisms by which changes of temperature exert teratogenic effects are discussed.

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The study of the embryological development of the heart and the viscera permits the establishment of an interrelation between the visceral situs, the malpositions of the heart, the truncoconal morphologies, and the position of the cardiac chambers; this in turn enables a correct classification and a precise nomenclature.Situs solitus, which is characterized by an anatomical right atrium, the greater lobe of the liver, and the suprahepatic segment of the inferior vena cava being placed on the right, is divided according to the direction of the apex into: (a) normally placed heart, with apex to the left, and (b) dextroversion, with apex to the right. Each one of these positions of the heart may in turn present with or without ventricular inversion. Situs inversus, characterized by an anatomical right atrium, greater lobe of the liver, and the suprahepatic portion of the inferior vena cava being placed to the left, is divided according to the position of the apex into: (a) mirror-image dextrocardia, with a right-sided apex and (b) laevoversion, with a left-sided apex. Each one of these positions of the heart may also have or not have ventricular inversion.The different malpositions of the heart in situs solitus or in situs inversus, with or without ventricular inversion with a right-sided apex or a left-sided apex, are divided into three subgroups according to the truncoconal morphology: without transposition of the great vessels, with transposition of the great vessels, and with common trunk.The diagnosis must be done carefully, establishing in the first place the visceroatrial situs and its relation to the direction of the apex in order to diagnose the type of malposition of the heart.If there are two vessels, their mutual relation is established, which permits the diagnosis of the type of truncoconal morphology; through it the spatialposition of the ventricles may be established by inference and, consequently, the atrioventricular relation.This paper describes the malformations associated with malpositioning of the heart, and anatomical examples of each type of malposition are presented.

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