Situs inversus, asymmetry, and twinning.

THE relation of situs inversus to twinning has attracted much attention from experimental embryologists. Thus, Spemann and Ruud (1922) produced situs inversus by division of the amphibian egg, the twin derived from the right half frequently showing situs inversus. Komai (1938) observed situs inversus in the salmon, particularly in one partner of conjoined twins and relatively frequently in the smaller twin. Naturally students of human twins have been interested in situs inversus in connection with the general problem of mirror-image asymmetry in twins. The relatively great dissimilarity observed in some human twins discordant for situs inversus has been considered as evidence of a mirror-image mechanism, the dissimilarity being supposed to be an indicator of the bilateral differentiation at the time when the division took place (Dubreuil-Chambardel, 1927; Cockayne, 1939; Helweg-Larsen, 1947). However, it can hardly be excluded that both the dissimilarity and the inversion in these cases are due to causes having no relation to the division or a mirror-image mechanism. In this connection, it is noteworthy that Mattison (1933) found only 1.3 per cent twin births among a total of 615 births occurring in the families of 4 individuals with situs inversus, compared with a frequency of 1.5 per cent twin births in Sweden generally. As this problem is a fundamental one in twin research, the following data which have been collected in a rather extensive study of situs inversus in Norway may be of interest.