Mechanisms of caudocephalic axis formation in the avian germ disc

We studied some mechanisms preceding or accompanying caudocephalic axis formation in the young avian germ disc. In an initial group this was done using eggs extracted from the uterus before oviposition. The development of germ discs after incubation in ovo (obliquely positioned or not) was compared with development after culture in vitro: germ discs develop much better when incubated in ovo than when cultured in vitro. We previously demonstrated that in obliquely-oriented germ discs, localized in situ on their egg yolk ball, the nucleus of PANDER and surrounding subgerminal ooplasmic layers present a spatial orientation parallel with RAUBER's sickle and with the temporally-predisposed sickle-shaped Anlage fields in the upper layer of the unincubated overlying blastoderm. The observations in our present study suggest that at the moment of bilateral symmetrization, a ``vertical'' influence emanating from the central subgerminal ooplasmic layers on parallel parts of the overlying blastoderm takes place. The first sign of the development of bilateral symmetry (already seen on the surface of a germ disc after 4 h of oblique positioning) is the appearance of a broad antisickle in the future cranial half of the blastodisc. In a second group, by excision of parts of unincubated or incubated blastoderms, followed by culture in vitro, we tried to explain some phenomena occuring during early axis development. Whilst gastrulation phenomena can take place after in vitro culture of isolated cranial quadrants of unincubated chicken blastoderms (stage X-XI of Eyal-Giladi and Kochav: 1976), this is not the case in isolated anti-sickle regions. The actual presence of neither RAUBER'S sickle material nor sickle endoblast is necessary for the formation of a primitive streak. Also the existence of a long-range secretion gradient of inducing factors derived from these structures after their removal can play an inducing role.