2 2 THE BIOLOGY OF TWINS 



umbilical cord; nor did the fetus make its appearance 

 until the boy was eight or ten years old, when, after 

 much enlargement of pregnancy and subsequent flooding, 

 the boy died." 



MODE OF ORIGIN OF DOUBLE MONSTERS 



Wilder in his monograph of 1904 considers that 

 double monsters are simply united or unseparated 

 dupHcate twins: 



To one who sees in separate twins the result of the total 

 separation of the first two blastomeres of a developing ovum there 

 is but one rational explanation of diplopagi, or those composite 

 monsters in which the two components are the duplicates of one 

 another and symmetrically united, namely, that here a similar 

 tendency to separation has been left incomplete, causing doubling 

 of those parts only in which the interrelations have been severed. 



Recently Wilder has abandoned his idea that duplicate 

 twins and double monsters are derived by the complete 

 or partial separation of the first two blastomeres, but 

 adheres firmly to the idea that each individual traces 

 back its cell lineage to one of those cells. 



Various other theories involving the idea of fusion, 

 as opposed to that of incomplete separation, have been 

 advanced. Fischer as early as 1866 supposes that 

 double monsters are the result of an early total fission 

 of the embryo followed by a secondary fusion of parts. 

 He says that they 



are invariably the product of a single ovum, with a single viteUus 

 and vitelline membrane, upon which a double cicatricula, or two 

 primitive traces, are developed. The several forms of double 

 malformation, the degree of duplicity, the character and extent 

 of the fusion, aU result from the proximity and relative position 



