54 A. A. SCHABFFER 



Now all these substances except uric acid are compound sub- 

 stances or mixtures of substances, and nothing is known of the 

 rate of solubility of the various ingredients in them as they 

 exist in the mixtures; it is therefore impossible to say just what 

 part of the mdia ink or the egg white or the very attractive car- 

 mine affects the sense organs of the ameba or in what manner. 

 Such reactions can shed but little light upon the problem of 

 the selection of food. 



It seems probable therefore that the use of isolated chemical 

 compounds is to be preferred to the use of mixtures of substances 

 in experimenting upon the feeding reactions of ameba because 

 in this way the number of unknown factors is reduced to the 

 minimum as far as the test substances are concerned. For this 

 reason a considerable number of experiments were performed 

 using as food substances globulin (crystalhn and ovoglobulin) 

 lactalbum n, ovalbumin, zein, keratin, and also a few compound 

 proteins such as gluten, aleuronat and fibrin. From the chemist's 

 practical point of view the isolated proteins mentioned can be 

 made quite pure and quite insoluble. Three of the compounds, 

 lactalbumin, ovalbumin, zein, were made under the direction of a 

 chemist with long experience in this line of work and these 

 products were made as pure as they can be made at present. 

 The other substances were bought from dealers in chemical 

 supplies. 



But the insolubility of these as of all substances is doubtless 

 relative, not absolute. The chemist, using his utmost skill, may 

 not be able to detect a minute fraction of a quantity of substance 

 going into solution, just as he is unable to detect odoriferous 

 substance in a hare's foot-tracks. But these small quantities of 

 material are frequently of the first importance to the student of 

 sense perception. So while we shall apply the term 'insoluble' 

 to globulin, lactalbumin, zein, ovalbumin, keratin, it should be 

 remembered that not only the word but the meaning also is 

 borrowed from the chemist. Speaking generally, we may not 

 expect to be able to understand the relation between a sense 

 organ and the substances which stimulate it by experimenting 

 with two or three substances which are said to be pure. It is 



