98 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



this point will give you the eight embryos you want. All 

 researches upon the development of isolated blastomeres 

 since the time of Herbst's discovery have been carried out 

 by this method, and it would have been quite impossible 

 by the old method of shaking to pursue the study into 

 such minute detail as actually has been done. It may 

 be added that calcium, besides its cell -uniting action, 

 is also of primary importance in the formation of the 

 skeleton. 



Among all the other very numerous studies of Herbst 

 we need only mention that potassium is necessary for the 

 typical growth of the intestine, just as this element has 

 been found necessary for normal growth in plants, and that 

 there must be the ion S0 4 , or in other terms, sulphur salts 

 present in the water, in order that the germs may acquire 

 their pigments and their bilateral symmetry. This is indeed 

 a very important result, though it cannot be said to be 

 properly understood. It is a fact that in water without 

 sulphates the larvae of Echinus retain the radial symmetry 

 they have had in the very earliest stages, and may even 

 preserve that symmetry on being brought back to normal 

 sea-water if they have spent about twenty-four hours in 

 the artificial mixture. 



We may now leave the subject of Herbst's attempts to 

 discover the morphogenetic function of the single con- 

 stituents of normal sea-water, and may devote a few 

 words to the other branch of his investigations, those 

 dealing with the morphogenetic effects of substances which 

 are not present in the water of the sea, but have been added 

 to it artificially. Here, among many other achievements, 

 Herbst has made the most important discovery that all 



