DEVELOPMENTAL HAZARDS OF HUMAN TWINS 137 



tion of symmetry in twins is discussed in a later chapter 

 of this book, we may postpone for the present a state- 

 ment of Spaeth's opinions on this subject. 



Schatz, who has written more extensively than any 

 other writer about human one-egg twins, holds quite a 

 different opinion from that of Spaeth as to the influences 

 of twins upon each other. This author had the advan- 

 tage of an adequate mass of data: an admirable collec- 

 tion of twin embryos and fetuses, together with their 

 fetal membranes, from the Marburg and Rostock gyne- 

 cological clinics. No other body of data on human twins 

 comparable to this has ever been brought together. In 

 several extensive tables Schatz gives lengths and weights, 

 together with percentage differences in weights and 

 lengths of twins. These are put into groups based on 

 the length and weight of the larger twin. Two classes 

 of twins are distinguished: 



A. Two-egg twins 



1 . Twins in which the two placentae are entirely separate 



2. Twins in which the two placentae are more or less fused 



B. One-egg twins (always with but one placenta) , 



The abundance of material enables the author to 

 compare the developmental differences of the two classes 

 of twins at various periods of pregnancy, instead of only 

 after birth, as Spaeth had done. This method reveals 

 the following important facts: 



a) The differences between two-egg twins, irrespec- 

 tive of whether or not the placentae are separate or 

 fused, increase steadily up to and after birth. 



b) The differences between one-egg twins are greatest 

 at about the middle of pregnancy and decrease steadily 

 until or after birth. 



