CoachCleavage ends in a blastula: a hollow ball (frog, sea urchin) or a disc (fish, bird) of a few thousand cells around a cavity, the blastocoel. Its cells are still unspecialised in appearance, but they are not equivalent: a fate map, made by staining patches of the surface with dyes and following them, shows that the animal cap will give skin and nervous system (ectoderm), the equatorial belt muscle, skeleton, kidney and blood (mesoderm), and the vegetal mass the gut and its organs (endoderm). These three germ layers are the same in every vertebrate, and the next step is to put them in their places: the mesoderm and endoderm are outside and must go in.
Examples
Example 10.10 (The timetable of a frog)
At : fertilisation; first cleavage at , then every half hour; blastula of cells at ; mid-blastula transition at ; gastrulation to ; neural folds close at ; heart beats at ; hatching and feeding at , as a tadpole of . A zebrafish does the same in one day, a chick in three, a mouse in nine; a human embryo gastrulates in the third week and closes its neural tube by the fourth — before most women know they are pregnant, which is why folate, needed for neural tube closure, is prescribed before conception.