This problem has been approached from quite a different angle. As is well known, certain bacteria require serum, blood, milk, etc., in order to grow satisfactorily on artificial culture media. It has been established recently, primarily by the work of Lloyd, that this peculiarity in the cultivation of microörganisms is due largely to their requirement of vitamine. She found a relationship of the inverse order between the amount of amino acid present in the culture medium and the amount of vitamine required to stimulate the growth of strains of meningococcus. Reasoning from this experience, she suggests that the action of the accessory growth factors is to increase the reaction velocity of the proteolytic metabolism. Here we find the vitamines once more regarded as catalyzers. This author, however, associates their activity with proteolytic rather than with carbohydrate metabolism. Interesting and suggestive work of similar nature has been carried out in relation to the growth of protozoa and of yeasts (Eddy). Investigations of this kind, dealing with unicellular organisms propagated on a simple food, have the advantage of greatly simplifying the problem. The recent work of Dutcher falls under this caption, differing merely in the fact that he attributes to the vitamines an _indirect action_. He has demonstrated that the tissues of polyneuritic birds show a decrease in catalase activity to a point 56 per cent. below normal, and that this activity is largely restored when the birds are cured with vitamine. According to this writer the vitamine functions as a metabolic stimulant, and its lack results in a depression of the body oxidations with an accompanying formation of toxic metabolic products, injurious to the nervous system. The action is regarded as coming about in an indirect manner, being accomplished through the hormone action of the vitamines on one or more glands of internal secretion. The _endocrine hypothesis_, suggested by Funk in his monograph, is not without some corroborative evidence. In testing the pharmacologic action of the water-soluble vitamine, Uhlmann found that it stimulated the various glands of the digestive tract, in this respect acting like pilocarpine. Some years ago Albert expressed the opinion that the action of this vitamine was "vagotropic" like atropine, and recently Dutcher has reported definite relief and cessation of polyneuritic symptoms by means of pilocarpine (0.5 mg. subcutaneously). He claims equally good results from thyroxin, the hormone of the thyroid gland, from desiccated thyroid and from tethelin (pituitary). Voegtlin and Myers conclude, as the result of experiments with brewers' yeast, that the chemical and physical properties of secretin and vitamine are identical.