Determination and cellular commitment in the embryonic amphibian mesoderm.

In classical embryology, the term 'determination' was used to describe the process by which a region of the embryo became committed to form a particular part of the body and it was understood that there was a particular stage of development at which determination occurred with respect to each decision. Some authors insisted that only an irreversible commitment should be described as determination, but this leaves undefined the nature of the preceding events involved in its establishment. We show here that there may be several stages of commitment for an organ rudiment that can be distinguished from one another by the use of different methods for altering the pathway of development. We have investigated the region of the amphibian mesoderm that is destined to become somitic muscle in the course of normal development and applied to it a variety of isolation and grafting techniques. The results, which show an extensive period of lability within this region, can be used to explain how pattern formation might occur within the dorsoventral axis of the mesoderm.