466 PROTOPLASM 



ments on rats). Extraordinarily large doses of vitamin D, 

 e.g., ten thousand or more times the therapeutic dose, result 

 in the deposition of calcium salts in all tissues of the body. This 

 can be carried so far as to mummify a rat. 



The final step in the study of irradiated ergosterol has been 

 its synthetic manufacture. This has been accomplished by 

 C. E. Bills, who converted ergosterol into vitamin D by treat- 

 ment with nitric oxide. The yield was only about 1 per cent. 

 The artificial and natural forms of vitamin D are not identical, 

 though the former has many of the properties of the latter. 



Three of the five substantial vitamins are associated with fats 

 — the growth vitamin A, the antirachitic vitamin D, and the 

 reproductive or fertility vitamin E. The first two occur in fish- 

 liver oils, the fat of egg yolk, milk, and green vegetables. Vita- 

 min E is found in butterfat, wheat germ, and lettuce (page 515). 



Other biologically important sterols are being determined to 

 such an extent that the fats, in particular the sterols, are assum- 

 ing great significance — far greater than heretofore realized — as 

 activators and organizers of bodily activities; indeed, they are 

 encroaching upon the proteins as the chief constituents of living 

 matter. In addition to the two sterols (cholesterol and ergos- 

 terol) so far discussed in regard to their close association with 

 several vitamins, there are numerous others or derivatives of 

 them which play very important parts in physiological reactions. 

 There is an isomere of ergosterol known as toxisterol which is 

 poisonous. Another is a heart poison. One of the bile acids 

 is a sterol or derivative. The male and the female sex hormones 

 are metabolic products of cholesterol with sterol characteristics 

 (the latter is androsterone) . The "organizer" in embryonic 

 development (page 510) is probably a sterol, and the cancer 

 producing hydrocarbon is a sterol derivative. 



Other fats, fatlike substances, and fatty acids have important 

 biological functions. Carrel and Baker found the growth- 

 inhibiting effect of blood serum on tissue cultures to be due 

 largely to hpoids. Diabetes appears to be due to an excess of 

 fatty acids. 



Fatty substances lower surface tension; they will, therefore, 

 aggregate at interfaces. This fact (the principle of Gibbs, 

 page 162) and experimental determinations of electric con- 

 ductance indicate that the surface layer of protoplasm is rich 



