DOUBLE MONSTERS IN FISHES 55 



ally symmetrical organisms: the now largely discredited 

 concrescence theory. 



In his more general work on the teratology of fishes 

 Gemmill (1872) adheres to his original idea of double 

 monsters as products of partial fusion of originally 

 separate embryos and accounts for the degrees of 

 separateness of the anterior parts in an ingenious way. 

 He considers the germ ring *'as a stock, able to give rise 

 vegetatively, so to speak, to more than one embryo.'* 

 As a rule only one embryonic shield arises on the germ 

 ring, but the germ ring is believed to be potentially 

 capable of giving rise to accessory embryonic shields 

 at any distance from the first. When in this way two 

 shields arise, the level of the point of union ''varies 

 directly with the original distance between the two 

 centers of embryo formation." 



In view of the fact that the validity or non- validity 

 of this ''budding theory" of twinning in fishes depends 

 on a proper interpretation of the nature of the germ 

 ring and the mode of formation of the embryonic axis, 

 the reader will doubtless be indulgent enough to allow 

 us to present a summary of the evidence on this point. 



THE CONCRESCENCE THEORY AND THE INTERPRETATION 

 OF CONJOINED TWINS IN FISHES 



According to the concrescence theory the embryonic 

 shield represents merely the head end of the future 

 embryo, while the lateral halves of the rest of the body 

 are separated from each other and are represented in 

 the germ ring. As the germ ring passes the equator of 

 the yolk it concresces to form the bilateral elements 

 of the embryo. This is evidently the view originally 



