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chondrial conversion of cholesterol to 5-pregnenolone is suggested as a rate-limiting step influenced by anterior pituitary trophins. There are probably similar mechanisms in the placenta. This organ, which contains a very active sulphatase system in the microsomes, can convert androgenic steroids (from the maternal and foetal adrenal cortex) into phenolic oestrogens. The last part of the book deals with current concepts of the mechanism of action of hormones. The genetic control of metabolism, protein biosynthesis and enzyme activity in cells are described and it is suggested that hormones may regulate a metabolic process essential to both replication of DNA and the synthesis of messenger RNA. In many tissues hormonal regulation is essential to cell function and replication, but it is possible that different types of hormones regulate cell function by different mechanisms. There is yet no clear evidence as to the mechanism of action of hormones at the cellular level. MAY REED