19 



cubator and in the protein-free filtrates tests for peptones — 

 biuret — and for amino-acids — ninhydrin — were made. The 

 proteins were precipitated by means of an excess of trichloracetic 

 acid and filtered off. 



The results of these tests are given in the following table: 



Table 1. 



Peptone and amino-acid tests in several digests of 

 starfish material. 



From this table three things are evident. In the first place 

 we see that qualitatively at least there is no difference between 

 the activity of the stomach digest and that of the radial sacs. 

 We will come back on this point in the chapter on the resorp- 

 tion in the starfishes. 



Secondly we see that gelatin is much better as a substrate 

 than egg-white, which does not astonish us when we remember 

 from human physiology, how relatively indigestible raw egg- 

 white is. 



In the third place we find that either the enzyme present 

 acts just as well in an acid medium as in an alcahne one, or 

 that Krukenberg is right in assuming the simultaneous pre- 

 sence of the two proteolytic enzymes. 



Therefore we must try to put the question on a more quan- 

 titative basis, which I did in the following way: 



11 '/2 gr- of radial sac and of stomach material were ground 

 up with sand. Equal quantities of gelatin, toluene and 100 c.c. 

 of water were added to each digest, one of which was acidified 

 as described above, the other one kept alcaline. After 24 days 

 they were removed from the incubator, the proteins pecipitated 

 with an excess of 5 "/q trichloracetic acid and filtered off. 



In the protein-free filtrate total-nitrogen determinations were 

 made on an aliquot, according to F o 1 i n's method. The Nessleri- 

 sation was direct. 



In the acid digest 25.4 mgr. of non-protein nitrogen were 

 present, in the alcaline one 144 mgr. 



This proves clearly that only one enzyme is present, i. e. 

 trypsin, which as we know, works over a relatively wide range. 

 If pepsin were present, we would expect much more non-protein 



