Studies on the cellular basis of morphogenesis in the sea urchin embryo. Directed movements of primary mesenchyme cells in normal and vegetalized larvae.

A time-lapse study has been made of the movements of the primary mesenchyme cells in the developing sea urchin larva. It shows that these cells move by pseudopod formation and contraction, and that a transition takes place--within a few hours--from a more or less random cluster, in the early mesenchyme blastula, to a well-organized, coherent pattern on the ectoderm of the gastrula. This organization is achieved by a striking random exploration of the wall of the larva by the pseudopods, followed by their contraction. The final pattern of the mesenchyme reflects those regions of the wall where the contacts between pseudopods and wall are most stable. The mechanism is thus one of selective fixation rather than of selective conduction. The pseudopodal contacts are seen to be continually made and broken, even when the final pattern is formed. The pseudopods of several cells may fuse to form a common pseudopod, these cells then migrating together. This is particularly evident in vegetalized larvae, but is also typical of the ventral side. Despite considerable variations in the way in which the final pattern is achieved, several main phases can be distinguished. The first is a radial displacement of the cells from the vegetal plate onto the presumptive ectoderm, followed by a phase of dispersion. The cells then gradually accumulate at a characteristic level, and form a ring. During this process, and when the ring is formed, the cells tend to accumulate in two clusters along the ring. The pseudopods of the cells in these clusters join into a cable, the end of which is highly branched; it explores the ectoderm, and extends the cell clusters to form branches from the ring. In vegetalized larvae, the pattern of distribution is simplified, but the same principles apply. It is suggested that the variations in the way in which the pattern is achieved are, in all probability, merely a reflexion of the lack of precision in the time sequence of changes in adhesive properties of the primary mesenchyme and blastocoel wall.