The contribution of the primitive streak to the somites in the avian embryo.

Chick embryos were removed from the egg at stages 6-11 and explanted in culture. The greater part of the postnodal primitive streak of each embryo was replaced with a similar region taken from a corresponding quail embryo. The reciprocal experiment was also carried out, chick primitive streak being grafted in place of quail. After further incubation, the grafted primitive streak cells were found to contribute to lateral plate mesoderm, somites and intermediate cell mass. In an additional series of experiments, the postnodal primitive streak was extirpated and the embryo allowed to heal without a graft being inserted; after further incubation, many more somites formed in these embryos. It is concluded therefore that the contribution of cells from the primitive streak shown in the first experiment may not be essential for somite formation. It is suggested moreover that two major morphogenetic movements are taking place simultaneously in the mesoderm during this period: one is the mediolateral migration of cells after ingression through the streak, whilst the other is an anteroposterior movement associated with regression.