§ 1] UPON THE RATE OF GROWTH 319 



the second culture, on the other hand, little growth had occurred even at 

 the end of five weeks. The crop from each of the cultures was then 

 harvested and its dry weight determined. That of the first culture was 

 0.3 gramme, that of the second 0.03 gramme. 



By a somewhat similar procedure MOLISCH ('95) has been 

 able to show that potassium is essential to the growth of algse ; 

 and NoBBB and others ('71) that it is necessary for the growth 

 of phanerogams. 



The experiments upon animals have been rather less satisfac- 

 tory on account of the greater difficulties in experimentation. 



Fig. 88. — Two embryos ol Sphaerechinus from parallel cultures. <x, reared in a solu- 

 tion containing all the necessary salts ; embryo normal, b, reared in the same 

 solution, but without potassium ; blastula wall abnormally dense, and embryo of 

 small size. (From Herbst, '97.) 



Nevertheless we have some trustworthy data upon this matter. 

 On the side of the invertebrates we have the experiments of 

 LoEB ('92), who placed a hydroid, Tubularia, in fresh water 

 to which solutions of various combinations of the salts found 

 in sea water were added so as to give approximately the nor- 

 mal osmotic effect. Under these circumstances regeneration 

 of the hydroids occurred only in the solutions containing 

 potassium. Again, Heebst ('97) finds potassium essential to 

 the growth of embryos of echinoids (Fig. 88). Thus the 

 potassium compounds seem necessary to the processes upon 

 which growth depends. On the side of vertebrates we have 

 the somewhat inconclusive results of Kemmerich ('69), who 

 fed two young dogs of the same age and of nearly the same 

 size upon meat boiled until a large portion of its salts was 

 extracted. To the food of one dog he added only sodium chlo- 



