254 Influence of environment on animals 



(c) On the production of twins from one egg through a change 

 in the chemical constitution of the sea-water. 



The reader is probably familiar with the fact that there exist two 

 different types of human twins. In the one type the twins differ as 

 much as two children of the same parents born at different periods ; 

 they may or may not have the same sex. In the second type the 

 twins have invariably the same sex and resemble each other most 

 closely. Twins of the latter type are produced from the same egg, 

 while twins of the former type are produced from two different eggs. 



The experiments of Driesch and others have taught us that twins 

 originate from one egg in this manner, namely, that the first two cells 

 into which the egg divides after fertilisation become separated from 

 each other. This separation can be brought about by a change in the 

 chemical constitution of the sea- water. Herbst observed that if the 

 fertilised eggs of the sea-urchin are put into sea-water which is freed 

 from calcium, the cells into which the egg divides have a tendency 

 to fall apart. Driesch afterwards noticed that eggs of the sea-urchin 

 treated with sea- water which is free from lime have a tendency to give 

 rise to twins. The writer has recently found that twins can be pro- 

 duced not only by the absence of lime, but also through the absence of 

 sodium or of potassium ; in other words, through the absence of one 

 or two of the three important metals in the sea-water. There is, how- 

 ever, a second condition, namely, that the solution used for the produc- 

 tion of twins must have a neutral or at least not an alkaline reaction. 



The procedure for the production of twins in the sea-urchin egg 

 consists simply in this: the eggs are fertilised as usual in normal 

 sea-water and then, after repeated washing in a neutral solution of 

 sodium chloride (of the concentration of the sea- 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% of the 

 eggs of Strongylocentrotm jmrpuratns 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 solu- 

 tion a mixture of sodium, magnesium and potassium chlorides (in the 

 proportion 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. 



