Rabbit blastocysts obtained 6 days after coitus were incubated individually or in groups for periods from 24 to 96 hr in a synthetic medium. Radioactive progesterone, pregnenolone, 17\ g=a\ \ x=r eq-\ hydroxypregnenolone, androstenedione and sodium acetate were used as substrates in these incubations. The blastocysts could biosynthesize cholesterol and pregnenolone from acetate, and promote biotransformation of the various steroid substrates. Addition of interstitial cell stimulating hormone and adrenocorticotrophic hormone to the medium increased the rate of differentiation of the blastocysts during in-vitro growth.

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