Practice question

Cleavage in amphibian embryos is holoblastic because moderate mesolecithal yolk does not prevent furrow progression, so entire egg divides. Division pattern follows radial symmetry, with first two cleavages meridional through animal-vegetal axis and third equatorial, displaced toward animal pole by vegetal yolk. Consequently blastomeres become unequal, with smaller, rapidly dividing animal cells and larger, yolk-laden vegetal cells. This holoblastic radial cleavage contrasts with meroblastic discoidal cleavage of birds and spiral cleavage of molluscs, maintaining regulative development until mid-blastula transition activates zygotic transcription.