The ectoderm is generally blue, the endoderm yellow, and mesoderm pink. The cells of the notochord are usually represented as green. You should try to follow the same system if you decide to make your own drawing of the embryos in lab. In contrast with amphioxus gastrulation, amphibian gastrulation is changed slightly due to the larger amount of yolk contained within the egg. Thus, the cells at the animal pole are retained for the formation of the embryo, while the yolk-filled cells at the vegetal pole are used more by the embryo as an energy source.
Gastrulation is initiated by invagination of cells to form the dorsal lip of the blastopore.
This is followed by pushing out of the ectoderm, and inward movement of the endoderm and yolk. The archenteron is formed and the blastocoel is slowly filled with cells and lost. For a short time the yolk-filled cells of the vegetal pole remain to fill the space between the dorsal and ventral lip of the blastopore, thus forming the yolk plug. After this stage, the gastrula then progresses into the neural tube formation stage, called neurulation. C. Gastrulation in birds With birds, we begin to see the meroblastic type of development of the embryos, and significantly more yolk than in the previous examples.