The correct interpretation of gastrulation is impossible without a knowledge of the position which the presumptive germinal layers occupy in the blastula. This position may be ascertained in various ways. A chart showing the fate of each part of an early embryo, in a particular blastula, is called fate map. The fate maps of different types of animal eggs can be constructed by following methods:
1. Construction of a fate maps by natural marking
The cytoplasm of the fertilized eggs of some animals such as some ascidians (e.g., Ciona, Styela (Cynthia) partita) has natural colour differences in the various regions. For example, on the basis of pigmentation differentiation, four general regions may be distinguished in the egg-cytoplasm of Styela partita (Cynthia partita): an upper hemisphere of light protoplasm, a yellow crescent postero-ventrally, a gray crescent antero-dorsally, and a vegetative area of dark gray yolky substance. Because of the natural differences in pigmentation, the developmental fate of these regions has been worked out with considerable precision. Knowing what these developmental prospects are, we may then view the areas as the components of a ‘map’ of prospective epidermal ectoderm, the yellow crescent is prospective mesoderm, dark grey yolky plasm is prospective endoderm, and grey crescent is prospective neural ectoderm and notochord.