138 THE PHYSIOLOGY OF TWINNING 



c) The result is that the two types of twins show 

 about equal differences at birth, a fact which is in agree- 

 ment with Spaeth's findings. 



d) At birth one-egg twins after a long period of 

 decreasing difference, and two-egg twins after a still 

 longer period of increasing difference, come to a period 

 of approximate equality. It follows from this that 

 during the whole period of pregnancy one-egg twins are 

 distinctly more different than are two-egg twins. And 

 this is a more striking circumstance in view of the fact 

 that their origin from one egg should tend to make them 

 more alike rather than more different. 



e) The only conclusion to be derived from these facts 

 is that the conditions of one-egg twinning tend to cause 

 one twin to have a pronounced effect upon the develop- 

 ment of the other. Schatz has made an exhaustive 

 study of the ways in which one twin may influence the 

 other. 



THE DISADVANTAGES OF TWINNING 



Before entering upon an account of the ways in which 

 human one-egg twins influence each other's develop- 

 ment, let us consider briefly some of the general dis- 

 advantages of twinning over single births. The human 

 uterus is of the simplex or undivided type and is adapted 

 for the really satisfactory gestation of but one fetus at 

 a time. When two or more fetuses come to occupy the 

 space usually filled by one, the twins, whether of the 

 one-egg or two-egg type, crowd each other and compete 

 for the common food supply. In the case of two-egg 

 twins it probably often happens that one egg reaches the 

 region of attachment first and tends to occupy the 



