Cleavage → blastula → gastrula correctly displays the sequence of developmental milestones
Step-by-step explanation:
In first step of developmental milestone, the zygote undergoes cell cleavage to form a spherical balls of cells, that are known a blastula. This blastula are filled with fluid called blastoderm at the centre. Later the balausta arranges itself in double layer out of which the inner one becomes the zygote and the outer one is placenta. After this stage the blastula folds itself into several layers i.e. germ layers which will give rise to the internal structure of living being. These germ layers are ectoderm, endoderm, mesoderm.