STRUCTURE AND DEVELOPMENTAL RATE 181 



pletely developed twins attached to the common yolk-sac. It 

 will be noted at once that each of the final twin individuals is 

 equally as large and perfect in form as is the single specimen at 

 the beginning of the series. This fact is of importance in showing 

 that up to this stage of development and growth there is no 

 question of available food, since the amount to be had in each egg 

 is here demonstrated to be sufficient to form two full-size perfect 

 young trout instead of the usual one. 



In studying the graded series of duplicities illustrated by plates 

 1 and 2, the question immediately presents itself as to why the 

 two components in the several specimens show the different 

 degrees of separation? What conditions or arrangements deter- 

 mined that the specimens in the upper part of the series should be 

 double headed, while those at the end of the series are com- 

 pletely double bodied? Gemmill ('12) in his monograph on the 

 teratology of fishes, considered these propositions and gave an 

 explanation for the varying degrees of doubleness which I believe 

 my studies completely confirm. On the other hand, Gemmill 

 failed to give any explanation of the initial or actual cause of 

 doubleness. 



In accordance with the view that has often been suggested, 

 the germ-ring was recognized by Gemmill "as a stock, able to 

 give rise vegetatively, so to speak, to more than one embryo." 

 The embryonic axis or body begins to form in the embryonic 

 shield which arises from certain places along the germ-ring. 

 When two shields arise, the degree of duplicity of the resulting 

 double fish "varies directly with the original distance between 

 the two centers of embryo-formation." When the centers of 

 embryo formation are close together, only 5° to 10° apart on the 

 germ-ring, the embryonic axes very soon become united so that a 

 double-headed specimen with a single body finally develops. 

 It may be stated generally that when the original buds are less 

 than 90° apart the specimens formed will exhibit various degrees 

 of double anterior halves on single posterior parts. When the 

 distance between the initial buds is greater than 90° and on up 

 to 180°, the resulting specimens will show the double condition 



