156 THE PHYSIOLOGY OF TWINNING 



acardia develops we may expect the injured component 

 of a pair of conjoined twins to exhibit the most pro- 

 found deterioration, equivalent to acormi, acephali, and 

 amorphi. Thus the injured individual may be reduced 

 to merely an extra abnormal head, an additional limb 

 or pair of limbs, or to a shapeless mass of more or 

 less differentiated tissues surrounded by a cyst. The 

 opportunity for complete suppression of the injured 

 twin is greater in the case of conjoined than in that 

 of separate twins, because of the fact that the two are 

 in such close contact. This relation makes it possible 

 for the tissues of the stronger component to grow more 

 or less completely around the weaker component and 

 to inclose it. Thus we may have a certain amount of 

 regulatory growth tending to obliterate the effects of 

 incomplete twinning. Cases have not infrequently been 

 observed in which an individual apparently quite nor- 

 mal has had removed from the abdominal cavity a 

 tumor which, on examination, has turned out to be 

 the amorphous remains of a formerly conjoined twin. 

 Such a regulation from conjoined twinning back to a 

 nearly normal single condition has been observed in twin 

 starfish larvae and has already been discussed (pp. 26, 27) ^ 



CONJOINED TWINS WITH SINGLE UMBILICAL CORDS 

 BUT SEPARATE HEARTS 



There seem to be very few cases of conjoined twins 

 that can be interpreted as having originally had only 

 one heart. If, in advanced stages, the autosite has a 

 heart and the parasite none we must conclude that the 

 latter is an acardius which has secondarily lost its heart. 

 Where there is only one umbilical cord and only one set 



