The growth and differentiation of post-implantation rat embryos together with their embryonic membranes, when grown in vitro in different blood sera, has been assessed by examination of the final stage of development attained and by the weight of protein synthesized. Rat embryos of early somite stages develop well to early limb bud stages when grown in homologous serum. This medium has some advantages over plasma clots. Maximum growth of the embryos is obtained in 0·5 to 1·0 ml serum/embryo. No advantage is gained by adding more serum, or transferring the embryos to fresh serum after a period in culture. It is immaterial whether the serum is obtained from male rats, pregnant or non-pregnant females, from the same individual as the embryos or some other, or from the same variety of rats as the embryos or some other.
Heterologous sera are unsatisfactory as culture media. Rabbit serum is rapidly lethal to rat embryos, but will support some development if pre-heated at 56 to 57° C for 30 min. Some samples of fowl serum are rapidly lethal to the rat embryos unless pre-heated, but others support some development.