DEVELOPMENTAL HAZARDS OF HUMAN TWINS 157 



of umbilical vessels it is obvious that we cannot account 

 for the inequality of the twin components as the result 

 of asymmetry in the third circulation of the placenta. 

 Such a condition could only be due to some asymmetry 

 or inequality in the bodies of the components them- 

 selves. Schatz considers that one of the chief sources 

 of initial inequality is partial situs inversus viscerum. 

 He has observed in double monsters not a few cases in 

 which a partial situs inversus of the heart or main blood 

 vessels causes unfavorable circulatory relations. In 

 complete situs inversus the components would doubtless 

 be equally favorably related to their respective body 

 parts, but a slight degree or any incomplete degree of 

 situs inversus will be unfavorable. Any initial handi- 

 cap, when there is competition for a single blood supply, 

 would doubtless result in progressive gain of advantage 

 on the part of the individual initially favored and a 

 progressive loss of ground by the less fortunate twin. 

 Thus might arise the conditions of autositism and para- 

 sitism. 



In a previous connection (p. 67), when discussing 

 the causes of inequality in the components of double 

 monsters in fishes, we offered as one explanation of 

 this inequality that there is probably as much vascular 

 anastomosis on the vitellum of the fish as on the common 

 placenta of human twins. Now that the reader has 

 seen how great an interinfluence actually does exist 

 between the components of conjoined twins and that 

 the mechanism of this interfluence is largely one involv- 

 ing unequal blood transfusion between the components, 

 he will appreciate the force of the argument against 

 Stockard's interpretation of the cause of the differences 



