It has been demonstrated that mouse blastocysts in delay, either lactational or following ovariectomy, are metabolically quite inactive (see McLaren, 1972). Although cell division continues in the first few days following the onset of delay (McLaren, 1968), mitotic figures are not seen in blastocysts held in delay for several days (McLaren, 1968; see Table 1). Just as trophoblast outgrowth characteristic of the implantation event may be reproduced in the appropriate culture media (Mintz, 1964; Cole & Paul, 1965), so too can delay of this event be achieved in vitro. Gwatkin (1966) has shown that omission of arginine and leucine from Eagle's medium can prevent trophoblast outgrowth from blastocysts for several days. Viability is apparently unaffected, as restoration of the missing amino acids to the medium after
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