Influence of Environment on Animals 205 



water), are placed in a neutral mixture of potassium chloride 

 and calcium chloride, or of sodium chloride and potassium 

 chloride, or of sodium chloride and calcium chloride, or of 

 sodium chloride and magnesium chloride. The eggs must 

 remain in this solution until half an hour or an hour after they 

 have reached the two-cell stage. They are then transferred 

 into normal sea- water and allowed to develop. From 50 to 90 

 per cent of the eggs of Strongylocentrotus purpuratus treated in 

 this manner may develop into twins. These twins may remain 

 separate or grow partially together and form double monsters, 

 or heal together so completely that only slight or even no 

 imperfections indicate that the individual started its career as a 

 pair of twins. It is also possible to control the tendency of such 

 twins to grow together by a change in the constitution of the 

 sea-water. If we use as a twin-producing solution a mixture 

 of sodium, magnesium, and potassium chlorides (in the propor- 

 tion in which these salts exist in the sea-water) the tendency 

 of the twins to grow together is much more pronounced than if 

 we use simply a mixture of sodium chloride and magnesium 

 chloride. 



The mechanism of the origin of twins, as the result of alter- 

 ing the composition of the sea-water, is revealed by observation 

 of the first segmentation of the egg in these solutions. This 

 cell-division is modified in a way which leads to a separation 

 of the first two cells (see Figs. 55 to 57). If the egg is afterward 

 transferred back into normal sea-water, each of these two cells 

 develops into an independent embryo. Since normal sea-water 

 contains all three metals, sodium, calcium, and potassium, and 

 since it has besides an alkaline reaction, we perceive the reason 

 why twins are not normally produced from one egg. These 

 experiments suggest the possibility of a chemical cause for the 

 origin of twins from one egg or of double monstrosities in mam- 

 mals. If, for some reason, the liquids which surround the 

 human egg a short time before and after the first cell-division 



