ONE-EGG TWINS IN FISHES 43 



duplicity than dorsal ones. Gemmill interprets this 

 condition as the result of the origin of two axes in 

 a single embryonic shield accompanied by secondary 

 fusion of dorsal structures. 



It should be emphasized that the last three modes 

 of twinning, as interpreted by Gemmill, are extremely 

 rare among fishes and that none of them have been noted 

 by those observers who have made large and represent- 

 ative collections. Cases of hemididymus have been 

 observed mainly in quite early germ-ring stages and there 

 is ground for believing that separations observed and 

 figured are merely cases of mechanical dehiscence more 

 or less temporary in character. The case of Barbieri, 

 cited above, is so extraordinary and so unlikely that, until 

 confirmatory instances have been described, it seems 

 hardly worth while to speculate about its significance. 



If we put aside the rare, exceptional, and poorly 

 understood types of duplicity in fishes we fiad that there 

 remain only two definite types of twinning: 



a) Separate twins, in which the entire bodies of the 

 two individuals are separate with the exception of 

 unions in the yolk-sac region. On account of mechanical 

 limitations it does not appear possible for fish twins 

 arising from a single blastoderm to be entirely separate. 

 Morphologically speaking, however, such individuals are 

 equivalent to separate one-egg twins in mammals and 

 will be considered here as separate twins. 



h) Conjoined twins of the anadidymus type, in which 

 there is anterior duplicity for a greater or less distance 

 and posterior singleness. This is the only type of fish 

 double monster described by those who have made large 

 collections. 



