1. Tuffrey, M., Bishun, N. P., and Barnes, R. D., Nature, 221, 1029 (1969).
2. Kirby, D. R. S., J. Embryol. Exp. Morphol., 10, 496 (1962).
3. Ford, C. E., in Tissue Grafting and Radiation (Academic Press, NY, 1966).
4. Uphoff, D. E., and Deringer, M. K., Coll. Intern. du Centre Nat. Rech. Scientifique, No. 147, 245 (1965).
5. Billingham, R. E., Palm, J., and Silvers, W. K., Science, 147, 514 (1965).