Spatial relationship between endophyll, primordial germ cells, sickle endoblast and upper layer in cultured avian blastoderms M. Callebaut, E. Van Nueten, H. Bortier, F. Harrisson, L. Van Nassauw and A. Schrevens Reprod. Nutr. Dev., 37 3 (1997) 293-304 DOI: 10.1051/rnd:19970305