1. As a result of the relatively simple operation of opening the yolk sac and thus exposing the foetal capillary circulation to flowing medium, it has proved possible to grow 12·5- and 13·5-day rat foetuses for a period of 42 h in culture.

  2. 25% rat serum, 75% Tyrode has been found to be a satisfactory and economic culture medium, and has the added attraction that foetal survival is improved compared with that obtained in whole rat serum.

  3. For both 12·5- and 13·5-day foetuses grown with open yolk sacs, a gas mixture of 95% O2, 5 % CO2 in equilibrium at 1 atm with culture medium flowing at about 15 ml/min has been found to give the best results.

  4. Under these conditions, foetuses explanted at 12·5 days increased their protein content from 1·0–1·3 mg to 2·3–2·7 mg during culture. Their somite number was 40–44 at explantation and reached 50–55 during culture.

  5. Foetuses explanted at 13·5 days increased their protein content from 3·2–4·0 mg to 6·1–7·5 mg during culture. Their somite number was 51–55 at explantation and reached 60–63 during culture.

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