166 CHARLES R. STOCKARD 



a. Arresting development by low temperature and the production of 

 double embryos and twins in Fundulus 



A number of years ago I occasionally found a double embryo 

 or a twin condition in Fundulus eggs that were arrested in their 

 development by being kept in solutions of jMgCl2 (Stockard, '09, 

 figs. 22, 56) and 57) . Such specimens, however, were so extremely 

 rare that their occurrence was never associated with the experi- 

 mental procedure. Chidester ('14) also found a twin among 

 Fundulus eggs arrested in ether solutions, and reported one 

 other in an egg which had developed in a crowded condition. 



The eggs of Fundulus heteroclitus are extremely hardy and 

 twins or double monsters are practically never found among 

 these eggs developing under ordinaiy conditions. During four- 

 teen spawning seasons many hundred control embryos have been 

 examined and I have not found among them a twin or double 

 specimen. While on the contrary trout eggs are known to be 

 rather sensitive, and must be developed under very carefully 

 regulated conditions. In the trout hatcheries double embryos 

 and twins are very often found and have at times been collected 

 and studied in large numbers (Windle, '95; Gemmill, '00, and 

 others) . 



Recently I have found strong evidence of a causal relation 

 between slowing development and the formation of twins in 

 trout, this will be discussed beyond. The evidence led me to 

 experiment with Fundulus eggs in order to determine whether 

 here also there was a direct connection between arresting devel- 

 opment or slowing its rate and the origin of double individuals 

 and twins. During the past three spawning seasons, a number of 

 experiments have been performed and the general results of these 

 may be reviewed. 



Two methods of slowing the rate of development have been 

 employed; lowering the temperature and reducing the oxygen 

 supply. The latter method will be considered along with the 

 occurrence of duplicities in trout eggs. 



It was soon learned that double embryos and twins could be 

 induced, but only by treating the eggs during a limited develop- 



