1. BIRGE, W.J., and J.J. JUST: Ntis rpt PB-232 075 (1973).
2. BROWN, D.D.: Carnegic Instn. Wash. Yearbook60, 440 (1961).
3. MILLER, J.C. and R. LANDESMAN: J. Embryol. exp. Morph. (in press) (1977).
4. NELSON, D.L. and E.P. KENNEDY: Proc. Nat. Acad. Sci. USA69, 1091 (1972).
5. NIEUWKOOP, P.D. and J. FABER: Normal table ofXenopus laevis (Daudin). Amsterdam: North-Holland Publishing Co. 1956.