180 CHAELES R. STOCKARD 



embiyos. In other words, some very small and simple chemical 

 or physical irregularity in the developmental environment is suf- 

 ficient to cause two embiyos to grow from the germ-ring, but is 

 not so injurious as to induce a deformed or abnormal develop- 

 ment in the young fish. When either component of these double 

 specimens is deformed, the cause of such deformities may be 

 more reasonably attributed to conditions other than the sur- 

 rounding environment (see beyond). 



Several years ago I obtained a large number of young trout, 

 many of which were twins and others presented different degrees 

 of doubleness. Since then I have visited several trout hatcheries 

 and have found in all that double specimens very frequently 

 occur. The practical fish culturists in two of these hatcheries 

 thought that such abnormal double specimens were caused by 

 early development under too crowded conditions, or in sluggish 

 water where the eggs did not obtain sufficient aeration. Such 

 views are very probably correct, since all of my experimental 

 studies with fish eggs has indicated that some retardation in rate 

 or interruption of development was the simple cause of unusual 

 structural responses in the embryo. Only recently, however, 

 could a satisfactory explanation of double conditions be worked 

 out on this basis, and the trout specimens gave the key to the 

 situation. The foregoing experiments with Fundulus were then 

 conducted to further substantiate the conclusions. 



The artificial production of double trout embryos is no doubt 

 rather difficult to bring about, since evidently only a slight slow- 

 ing of the rate of cell proliferation at a particular moment is 

 favorable. 



Plates 1 and 2 illustrate a series of double trout which are 

 selected from the large number of such specimens that have been 

 obtained. The series shows the various degrees of double forma- 

 tion, beginning with a partially double-headed condition, and 

 passing through the double anterior regions on single bodies, to 

 double bodies with single tails, and on to the condition of com- 

 plete doubleness but with the two components jointed more or 

 less intimately together. The final specimen shows two com- 



