Morphogenetic movements in Salmo gairdneri Richardson

Uncoordinated movements of individual internal blastomeres of the pregastrular blastodisc are distinguished from true morphogenetic movements which later segregate the germ layers and establish the embryonic axis and its organs. These coordinated movements are described in detail for stages 7 through 10 on the basis of recorded drift of chalk dust implanted among the blastomeres, and of vital dye spots printed on the blastodisc. The movements of the chalk and of the living colored cells follow the same pattern but the chalk gives more precise information over a longer time span. There is no evidence of invagination, or in‐wheeling, or centripetal convergence. Most of the movements combine epiboly and convergence, but in different proportions varying with the depth of the cells and their distance from the blastodisc center. A line of chalk dust, implanted by needle thrust down the full depth of the blastodisc often divides into a superficial, chiefly epibolic, cloud of granules and a deeper one, convergent toward the axis. Particles from a single implantation site may become incorporated into a variety of organs and tissues, indicating that a fate map for Salmo must be three‐dimensional. The forebrain receives secondary contributions from the center of the blastodisc. Movements directly comparable to closure of neural folds are described during formation of the brain. The role of the germ ring is examined in detail. It does contribute to the mesoderm of the tail. Migration along it toward the axis involves only the hypoblast, through which cells move at very different rates.