4 THE BIOLOGY OF TWINS 



The one fact that stands out above all others in this 

 connection is that there are really but two distinct 

 kinds of twins: those that come from a single egg and 

 those that come from two or more eggs. The former 

 type involves some process of fission or division of a 

 germ, which is at first a single individual, into two or 

 more complete or, in certain cases of incomplete twin- 

 ning, partial individuals. This is really the only true 

 type of twinning. 



When, however, twins or triplets, etc., are derived 

 from two or more eggs, the biology of the situation 

 differs very little from the ordinary phenomenon of 

 multiple births, as seen in swine, dogs, cats, and other 

 common mammals. In these animals we do not use the 

 term twin, triplet, or quadruplet, because this giving 

 birth to a number of offspring at a time is the normal 

 condition, whereas a single offspring is exceptional. It 

 is only in those species that normally give birth to but 

 one offspring at a time that we note especially the more 

 or less frequent exceptions to the rule, and refer to 

 double births as twins, triple births as triplets, quadruple 

 births as quadruplets, etc. 



Monozygotic twinning, where a single egg produces 

 plural offspring, is therefore a phenomenon that should 

 be considered as only a phase of the much more general 

 phenomenon of symmetrical division. The develop- 

 ment of the right- and left-hand homologous organs 

 in a bilateral organism is essentially a twinning process, 

 for it involves the division of a median unpaired pri- 

 mordium into two equivalent parts, one of which is 

 the mirror-image of the other. In many cases this 

 twinning of parts may be more or less completely 



