I40 THE PHYSIOLOGY OF TWINNING 



twins and there seems to be no difficulty in distinguishing 

 one-egg from two-egg twins. A study of the detailed 

 anatomy, especially the vascular anatomy of twin 

 fetuses, together with that of the fully injected placen- 

 tae and umbilical blood vessels, enables the author to 

 determine the probable mechanism of the interinfluences 

 of one-egg twins. Leaving out of consideration all injuri- 

 ous conditions which one-egg twins may have in common 

 with two-egg twins or with single fetuses, let us focus 

 our attention upon those interinfluences peculiar to one- 

 egg twins. 



Schatz distinguishes two types of fetal interinfluence 

 incident to one-egg twinning. The first is associated 

 with situs inversus viscerum or the possession by one of 

 the twins of an asymmetry of the heart, stomach, and 

 viscera which is the mirror-image of that of the other 

 twin or of that characteristic of the species. This 

 reversal is conceived of as a direct result of the twinning 

 process, though there is no definite theory to account 

 for it. All degrees of inverse symmetry are noted, 

 ranging between a slight degree of it to complete reversal. 

 It is very common in conjoined one-egg twins and 

 relatively rare in separate one-egg twins. Schatz con- 

 siders that inverse symmetry of the blood vessels, 

 especially when the reversal is slight or incomplete, has 

 very serious consequences for the unfortunate twin in 

 which the inverse symmetry exists. Among other things, 

 it may lead to a bad connection with the umbilical blood 

 supply, which remains normal in its relations. Schatz 

 enters into a detailed discussion of the secondary effects 

 upon the vascular system of a fetus in which inverse 

 symmetry exists and cites a number of cases of badly 



