44 THE PHYSIOLOGY OF TWINNING 



These two standard classes of fish twins will now be 

 considered in detail. 



SEPARATE TWINS 

 MODE OF ORIGIN 



All of the writers who have attempted to formulate 

 theories of the mode of origin of fish twins in general 

 have offered the same explanations for both separate and 

 conjoined twins. Gemmill's view is representative of 

 all such views and is expressed as follows: 



The recorded observations indicate that double-monster 

 fishes (including those united by yolk sac only) always arise on a 

 single yolk and from a single blastoderm at the margin of which 

 two more or less separate centers of gastrulation and embryo- 

 formation have appeared. 



The twin centers of embryo-formation mentioned above may 

 be classed in two groups {a) and (b), according to the distance 

 which separates them from one another, (a) In the first and most 

 important group the interval is not too great to prevent approxima- 

 tion and union of the two embryonic axes from taking place 



during the natural course of their growth in length (^) In 



the second group, the twin centers of embryo-formation are so 

 far apart that there is no compelUng influence of the kind described 

 above which would lead to the approximation and union of their 

 growing embryonic axes. Accordingly the twin bodies remain 

 separate, except for the adventitious union supplied by the 

 layers forming the common yolk sac. 



I see no reason to doubt that at least many of the 

 truly separate fish twins arise in the manner described 

 above. But I have reason to believe that at least some 

 separate twins and all truly conjoined twins arise by 

 partial dichotomy or by complete fission of the right- and 

 left-hand primordia of a single axis. In other words, 

 there may be two types of separate twins, those that 



