146 THE PHYSIOLOGY OF TWINNING 



arterial blood pressure; intra-uterine opening up of the 

 pulmonary circuit; thickening of the walls of the blood 

 vessels, especially of the arteries; hypertrophy of kid- 

 neys and bladder; excessive urine; and excessive am- 

 niotic fluid. 



All of these changes are decidedly detrimental and 

 have, in most cases, caused the premature death or 

 abortion not only of the injured, but of the favored 

 twin. Only those cases in which a marked difference 

 between the twins existed have been made the object of 

 special study; yet one cannot help but suspect that, even 

 in twins that are nearly equal, go to full term, and live 

 for a considerable time after birth, some of the after- 

 effects of minor degrees of the changes listed above may 

 persist in one or both twins and help to account for the 

 lower vitality and earlier death of one twin or the pre- 

 mature death of both twins. 



Serious as are the effects of blood exchange for the 

 favored twin, they are trifling as compared with those of 

 the injured twin. 



THE INFLUENCE OF ASYMMETRY OF THE THIRD CIRCULATION UPON 

 THE DEVELOPMENT OF THE INJURED TWIN 



The primary effect upon the injured twin is a diminu- 

 tion of the blood supply. In cases of only slight inequal- 

 ity it is likely that a somewhat lessened blood supply 

 is a healthier condition than one somewhat increased. 

 The after-effects are not so likely to be serious. In 

 cases of marked inequality, however, the consequences 

 are for the injured twin very serious indeed, because 

 nutrition is so reduced that development is either 

 entirely arrested or certain particular structures fail to 

 emerge from the fetal condition. In less pronounced 



