A second thing to be mentioned about growth is a discovery which has attracted a great deal of attention of late years; we will realize of course that the chief thing in the making of new protoplasm is the building together of protein out of amino acids. It is evident that for the manufacture of new protein all the eighteen amino acids must be present in sufficient proportion to give enough of each. There are some plant proteins which lack one or two of the amino acids that are present in human proteins and when a growing animal goes on a diet in which these are the only proteins present it at once stops growing. Most of the experiments proving this have been done on white rats, and it has been found possible to keep a rat for more than a year at the size it was when only a few weeks old simply by feeding it proteins in which one or two amino acids were lacking. The fact that the animal lived during this time proves that his protoplasmic wastage was made good and therefore that there are proteins which can replace the body wastage but cannot manufacture new protein. There is only one conclusion to be drawn from this, namely that the daily wastage which the protein suffers does not include all of it; some amino acids when once built in are there for good; others, on the other hand, are constantly being lost in the process of wastage, and these are the ones which must be replaced. There is a common protein, gelatin, which is used a great deal for food, but which by itself will not serve either for repair or growth because it not only lacks some of the amino acids that are in living protoplasm, but also some that are lost in the process of wastage. Gelatin is useful as a food, therefore, only in combination with other proteins that contain the necessary amino acids; or after the nitrogen has been taken out, it becomes a good fuel.
One interesting question that was settled by the experiments described above was whether the ability to grow is exclusively a matter of youth; as we know, under ordinary conditions only young animals grow. When they reach a certain age, they become mature and thereafter no increase in size takes place. In the case of these white rats which were kept for more than a year at the size of partially grown rats it was found that as soon as their diet was changed to one in which all the necessary amino acids were present they would begin at once to grow and grow to full size, when they would stop growing just as they would have when young. This proves that growth is not a matter of age, but is a matter of achieving a certain size, and is controlled by factors which we do not at present understand. An animal that has not been able to attain