III. 8 CHEMICAL COMPOSITION 141 



Herbst has not omitted to point out the significance of these 

 and indeed of all monstrosities for the theory of the origin of 

 those larger, discontinuous variations krTown as ' sports ', or, in 

 more modern phraseology, ' mutations ' ; and Vernon has been 

 able to show statistically that the degree of continuous variation 

 may also be altered by changes in the chemical environment. 



In all the foregoing experiments the effect is observed of the 

 addition of some chemical substance to the medium in which 

 the embryo is placed. We have now to consider a very remark- 

 able series of investigations, for whose planning and execution 

 we are indebted to the genius of Curt Herbst, investigations in 

 which substances which are present in the normal environment 

 of the larva are omitted, and an insight thus gained into the 

 part they play, if any, in the normal development of the 

 organism. Herbst has indeed succeeded in demonstrating in 

 the most conclusive manner the necessity to the sea-urchin egg 

 for the normal performance of this or that phase of develop- 

 mental function of a large number of the elements present in 

 sea-water. 



The sea-water at Naples, where Herbst carried out his work, 

 has the following composition : 



NaCl . . , .3% 



KC1 ..... .07% 



MgCl 2 . . . . \ -32% 



MgSO 4 . . . . . -26% 



CaSO 4 . ... . -1% 



CaHPO 4 \ 



CaCO 3 

 Fe 2 CO 3 



Si 

 Br 



I 



in small quantities. 



It may be said at once that silicon, bromine, and iodine are 

 unnecessary, and that, though earlier experiments led Herbst 

 to believe that phosphorus and iron were essential, he has since 

 assured himself that phosphorus is certainly, and iron probably, 



