Early mesoderm differentiation in the chick embryo.

Two groups of experiments were carried out. In the first group, grafts of quail mesoderm whose presumptive fate was to form somites or heart tissues, were taken from quail embryos (stage 4-5 of Hamburger and Hamilton 1951) and inserted beneath the ectoderm of chick embryos of stage 3-4 immediately lateral to the primitive streak. Whilst most grafts contributed to the somites and or the heart, 22 out of a total of 46 were found to have contributed also to the pharyngeal endoderm. Although three of these grafts were known to have included some quail endoderm cells, the remainder were considered to consist of mesoderm alone. It is concluded that mesoderm at the primitive streak stages is still capable of forming endoderm. In the second group of experiments, grafts of quail somites (stage 10-14) were inserted beneath the ectoderm of chick embryos of stage 3-4. In 18 out of 23 cases the graft cells were found in somitic tissue, but they were also found in the endoderm (4 specimens), lateral plate (3 specimens) and endothelium (4 specimens). It is concluded that even at stages 10-14, the somite-derived cells are still not completely determined to form somite derivatives. In those cases where the grafted somites differentiated further, sclerotome cells which migrated from them did not necessarily move towards the host notochord.