42 



LIFE: ITS NATURE AND ORIGIN 



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Figure 3. Skeleton models of the twenty possible stereoisomers of /3-carotene: 

 all -^ram-/3-carotene, three mono-c/s-/3-carotenes, six di-ciV/3-carotenes, six tri-cis-/3- 

 carotenes, three tetra-ci's-/3-carotenes, and all-cfs-/3-carotene. (Courtesy Chemical Re- 

 views) 



