centrolecithal

Centrolecithal refers to a pattern of yolk distribution in eggs where most of the yolk is concentrated at the center of the oocyte, with only a thin layer of cytoplasm surrounding it. This arrangement is typical of many insect eggs and some other arthropods. The centralized yolk makes complete holoblastic cleavage difficult, so cleavage is meroblastic or superficial.

In centrolecithal eggs, early cell divisions occur in the peripheral cytoplasm around the central yolk. The nuclei divide rapidly without cytokinesis, forming a syncytial (multinucleate) layer beneath the yolk. As divisions proceed, nuclei migrate toward the periphery, and eventually cellularization occurs, producing individual cells that comprise the blastoderm. The embryo then develops through the usual successive stages of gastrulation and organ formation, but the initial pattern is shaped by the central yolk.

Examples and significance: centrolecithal eggs are exemplified by several insects, including Drosophila melanogaster, where rapid, superficial nuclear divisions create a syncytial blastoderm before cellularization. This mode of cleavage allows fast early development despite the presence of a large central yolk and contrasts with holoblastic cleavage seen in isolecithal or lightly yolked eggs. The term is part of a broader framework that classifies embryos by yolk distribution and its impact on cleavage and early development.