meroblastic

Meroblastic refers to a mode of embryonic cleavage in which the zygote divides only partially, because a substantial amount of yolk is present in the egg. In meroblastic cleavage, cytoplasmic divisions are restricted to a portion of the fertilized egg, while the yolk mass remains largely undivided. This form is typical of telolecithal eggs, such as those of birds and many reptiles and fishes, and it leads to the formation of a blastodisc or a peripheral cell layer rather than a completely divided zygote.

Two main patterns are described: discoidal meroblastic cleavage and superficial (or syncytial) meroblastic cleavage. Discoidal meroblastic cleavage occurs when only a small disc of cytoplasm at the animal pole divides, producing a blastodisc on top of the yolk; the remainder of the yolk stays undivided. Superficial meroblastic cleavage occurs in eggs with very large yolk, such that nuclei divide without cytoplasm dividing, forming a syncytial layer beneath the eggshell or periphery; later cellularization yields a blastoderm.

Meroblastic cleavage contrasts with holoblastic cleavage, in which the entire zygote divides in each cell cycle, as seen in isolecithal eggs of many amphibians and mammals. The mode of cleavage influences early development, including the timing of gastrulation and the arrangement of embryonic tissues. In many insects, cleavage is also meroblastic but often described as superficial, with subsequent cellularization forming the embryo.