INTRODUCTION 743 



The terminology introduced by Askew and co-workers^' "''^ and by Win- 

 daus e( al.^^'^^'^'^ has persisted in the Hterature until the present. Two 

 active fractions were obtained by distillation of irradiated ergosterol. The 

 first of these, which Windaus called vitamin Di, was later shown by the 

 same worker, in collaboration with Dithmar and Fernholz,*^^ to be impure; 

 it consisted of the active vitamin D with an inactive decomposition prod- 

 uct, lumisterol. Askew also found that one of his fractions consisted of the 

 active vitamin Di, calciferol, combined with two inactive components, one 

 of which was lumisterol. As a result of this confusion, Windaus suggested 

 the retention of the designation, vitamin D2, for the active vitamin D ob- 

 tained by irradiation of ergosterol; it was recognized that this is identical 

 Avith calciferol. No one has suggested a revision in the nomenclature since 

 that time, and the D vitamins consist of a group of substances in which the 

 first member, vitamin Di, does not exist. The irradiation product of 7- 

 dehydrocholesterol'^* has been termed vitamin D3 by the Windaus group."'* 

 The activated product obtained by Windaus and Langer,*^ from 22- 

 dihydroergosterol, after synthesis of the latter, has been designated as 

 vitamin D4. 



As early as 1935, Bills™ recognized that vitamin D exists in multiple 

 forms. The figure of six different active compounds which was first pro- 

 posed™ was later changed to eight.'' The fact that these are frequently 

 associated was demonstrated by the results of Bills et alP obtained with a 

 variety of fish liver oils, which were found to have widely varying potencies 

 when tested on rats or on chickens. 



It is now generally recognized that a number of additional products can 

 be included in the category of provitamins D, although none of the acti- 

 vated forms have potencies approaching those exhibited by vitamins D2 

 or D3, or even by vitamin D4. The added provitamins D include 7-dehy- 

 drositosterol,'^^ 7-hydroxycholesterol, and 7-dehydrocampesterol, which has 



«i F. A. Askew, R. B. Bourdillon, H. M. Bruce, R. G. C. Jenkins, and T. A. Webster, 

 Proc. Roy. Soc. London, B107, 76-90 (1930). 



" F. A. Askew, R. B. Bourdillon, H. M. Bruce, R. G. C. Jenkins, and T. A. Webster, 

 Proc. Roy. Soc. London, BlO?, 91-100 (1930). 



" T. C. Angus, F. A. Askew, R. B. Bourdillon, H. M. Bruce, R. K. Gallow, C. Fischer- 

 mann, J. S. L. Philpot, and T. A. Webster, Proc. Roy. Soc. London, B108, 340-359 (1931). 



«< F. A. Askew, R. B. Bourdillon, H. M. Bruce, R. K. Callow, J. S. L. Philpot, and 

 T. A. Webster, Proc. Roy. Soc. London, B109, 488-506 (1932). 



«* A. Windaus, A. Liittringhaus, and M. Deppe, Ann., 489, 252-269 (1931). 



«6 A. Windaus, F. v. Werder, and A. Liittringhaus, Ann., 499, 188-200 (1932). 



" A. Windaus, K. Dithmar, and E. Fernholz, Ann., 493, 259-271 (1932). 



68 A. Windaus, 11. Lettre, and F. Schenck, Ann., 520, 98-100 (1935). 



«9 A. Windaus, F. Schenck, and F. v. Werder, Z. physiol. Chem., 24I, 101-103 (1936). 



•" C. E. Bills, Cold Spring Harbor Symposia Quant. Biol, 3, 328-340 (1935). 



^' C. E. Bills, Paper read at convention of the .American Medical Association, Mav 14, 

 1936. Cited by C. K. Bills ei al, J. Nutrition, 13, 435 (1937). 



" C. E. Bills, (). N. Massenguie, M. Inil)oden, and II. Hall, ,/. Nutrition, 13, 435-452 

 (1937). 



" W. Wuiiderlich, Z. physiol. Chem., "241, 116-124 (1936). 



