378 FRANK R, LILLIE 



my principal work has been to secure this evidence, which is 

 herewith presented. The order of presentation is decided by the 

 theoretical considerations, which obviously require that we should 

 first of all determine whether we are dealing with a phenomenon 

 of division of a single zygote, or with development of two zygotes 

 in juxtaposition. Other considerations follow immediately from 

 this, and the anatomical side is considered last. In conclusion 

 the more fundamental theoretical questions come up for con- 

 sideration. 



II. ARE THE STERILE FREE-MARTIN AND ITS MATE MONOZYGOTIC 



OR DIZYGOTIC? 



The first question that confronts us, therefore, is whether the 

 free-martin and its male twin are monozygotic or dizygotic? 

 This question can be answered decisively only by the embryo- 

 logical data; contributing evidence may be furnished by study 

 of the degree of resemblance of the twins to one another, and 

 by a statistical study of the sex ratios in twin births in cattle. 



1. The embryological evidence 



There are two ways in mammals of deciding whether twins are 

 monozygotic or dizygotic: 1) If there is a single chorion for both 

 of the twins, this would usually be regarded as evidence of mono- 

 zygotic origin. But a single chorion is not decisive evidence, 

 because, though one would expect monozygotic twins to have a 

 single chorion, yet it is theoretically possible that two separate 

 chorions may fuse to form one. The monochorial condition 

 may be primary or secondary. 2) The number of ova concerned 

 in a pregnancy may be ascertained by the number of corpora 

 lutea, which correspond accurately. If we should find two 

 corpora lutea for all free-martin twins, the dizygotic origin would 

 be proved, provided that the rule holds for cattle. 



I have had the opportunity of examining a large number of 

 twin pregnancies in cattle through the courtesy of Messrs. Swifl 

 and Company of the Chicago Stockyards; and I wish to express 

 my appreciation of their generous cooperation, without which 

 this study could not have been made. The superintendent of 



