ABSTRACT
I. INTRODUCTION A fascinating aspect of development is the rapidity with which increasingly complex forms develop from relatively amorphous structures. In the chick embryo there is an almost hourly progression as the neural tube closes in a cranialcaudal direction and the strip of unsegmented mesoderm on either side breaks up into a regular series of somites, prefiguring the segmental appearance of the overt body plan (peripheral nervous system, vertebrae, and axial muscles). A short time later limbbuds form as simple outpockets from the body containing undifferentiated, loosely aggregated mesenchyme cells, only to transform within several days into complex structures composed of cartilage, muscle, and connective tissue organized in a highly precise pattern. During the same period neurons grow out of the central nervous system to establish a precise set of connections with specific muscles, so that the embryo begins to move in a coordinated way. By what mechanisms does such complex form arise, and which aspects are most suited for study in the avian embryo?
