204 The Mechanistic Conception of Life 



beyond the metamorphosis into the adult stage and since in all 

 the experiments made by the writer the parthenogenetic plutei 

 lived as long as the plutei produced from fertilized eggs. 



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 

 tj^pes 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 fertilization 

 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 fertilized 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 afterward 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 produced 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, however, a second condition, 

 namely that the solution used for the production 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 fertilized as usual in 

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

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



