II. CHEMISTRY 



137 



ring systems are identical and exhibit but one double bond, located at C-5. 

 These sterols and the dihydro derivatives which accompany them in traces 

 lack any system of conjugated double bonds and hence do not show ab- 

 sorption bands in the ultraviolet region when examined spectrographically. 



TABLE I 

 Sterols of Mollusks (Bergmann--) 



On the other hand, the sterols which are most characteristic of the lower 

 forms of life and which occur in traces with the sterols of the higher forms 

 (ergosterol, 7-dehydrocholesterol) have double bonds at C-5 and C-7. As 

 would be expected on theoretical grounds, these sterols with conjugated 

 doul)le bonds exhibit ultraviolet absorption bands. With these principles in 

 mind, one can look back understandingly on the development of the pro- 

 vitamin theory. 



