Passage I (Questions 1-6)

    Gastrulation is one of the most dramatic and crucial stages of vertebrate embryogenesis. The embryo, at first simply a hollow ball of cells, undergoes a transformation into a multilayered structure with a central gut tube and bilateral symmetry. The outer layer of cells formed during gastrulation is considered ectoderm, while the middle and inner layers are considered mesoderm and endoderm, respectively. Since interactions between these three cell layers will determine the further developmental fate of the embryo, events occuring during gastrulation are vital for the proper development of the organism.

    Gastrulation begins when cells around the embryonic blastopore begin to invaginate, or move towards the inside of the embryo. The site where this invagination initiates is referred to as the dorsal lip of the blastopore. In the first half of this century, it was learned that the dorsal lip acts as an organizer, inducing the tissue directly around it to begin invaginating while triggering tissue further away to adopt other specific cell fates.

    It was later found that the cells of the dorsal lip secrete a diffusible signalling substance that slowly degrades after its secretion. The result is a concentration gradient of the substance, with higher concentrations existing closer to the source of the secretion. Cells at different distances from the dorsal lip are exposed to different concentrations of the substance, leading to the signalling of different behaviors. Due to its effect on morphogenesis (the "shaping" of the embryo), this substance was referred to as a morphogen.