Abstract
It is known that the situs of the heart and viscera can be experimentally reversed by turning around a portion of the archenteric roof in the open neural plate stage of development. 1 Spemann and Falkenberg 2 have also shown that if salamander eggs are divided into 2 by a hair loop, the sinistral larvae which result all have normal situs while some of the dextral ones have situs inversus. Spontaneous double monsters in man as well as in some of the lower vertebrates occasionally show situs inversus of the heart and viscera of the right member of the pair (A component). 3 Politzer, 4 however, reports 2 cases of spontaneous twinning in Salamandra in which situs inversus occurred in the left member (B component). Swett 5 has also reported situs inversus viscerum in the left member of double trout. Moreover, in some spontaneous dicephalus monsters of Rana sylvatica discovered some years ago, the hearts of the left individuals were in inverse situs, while the right hearts were of normal asymmetry.
This paper is a preliminary report of an attempt to study the question further. Parabiotic twins were made from Rana pipiens embryos in open neural plate stage by removing the ectoderm, and in some cases the mesoderm as well, from a portion of the lateral body wall and holding the embryos together until healing took place. Fifty-eight pairs were available for study at the feeding stage.
Although in most cases nothing but the ectoderm had been removed in making the twins, and the 2 hearts lay in a common pericardium with the sinus venosus of each animal communicating with that of the other, the right ventricle was larger than the left in 36 cases. The ventricles were the same size in 6, and the left ventricle larger in only 11 cases.
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