The chapter on amino-acids (page 52) has shown that different proteins have not the same biological value. Casein, for example, is a superior protein to gelatin. In the preparation of "synthetic diets" casein is invariably used. The question now arose as to what would happen if in the place of casein we substituted gelatin?

Five rats per set. Weight on the first day:

A 50 B 53

For the first four days both A and B received the regular diet. On the fourth day the weights were

A 61 B 63

A was now placed on the "casein synthetic" and B on the "gelatin synthetic." The weight on the fifth day was:

A 64 B 65

Then B began to decline. A, however, continued to increase until the 20 day. The weights then were:

A 83 B 51

At this point meat extract6 was added to the diet of B. This did not improve matters. On the 37 day the weights were:

A 71 B 43

Meat extract was next added to A's diet. On the 44 day the weights were:

A 65 B 41

Evidently nothing in meat extract had any beneficial effect on either group.

But now comes the crucial experiment. To the daily diet of each a small quantity of yeast (one-half a gram of the dried material) was added. The gelatin-fed animals responded slowly, but A recovered very rapidly; on the 55 day the weights were:

A 96 B 45

6 This is the well-known Liebig 's beef extract, which contains very little of the protein and fat of the original beef, but does contain small quantities of nitrogenous substances such as creatine, and xanthine and hypoxanthine, that impart to it its stimulating properties.

Now the regular diet was given to both sets. On the 61 day:

A 123 B 74

This experiment is an instructive one. In the first place it confirms the superiority of casein over gelatin (as protein); in the second place, it proves that meat extract, usually given in the form of soup or Liebig's extract, does not improve conditions, despite its content of such important nitrogen compounds as creatine, xanthine and hypoxan-thine; in the third place, it shows the remarkable influence exerted by a small quantity of yeast, due presumably to its vitamine content, when added to a "synthetic diet" containing casein; in the fourth place, it presents evidence to show that yeast has little effect when added to a "synthetic diet" in which the protein is represented by gelatin: and in the fifth place, it shows how remarkable is the recovery of the "gelatin-synthetic" animals when they are placed upon a regular diet, showing clearly enough that not only were these animals in need of vitamine, but equally in need of the proper kind of protein, among other things.