yolkcontaining

Yolkcontaining is an adjective used in biology to describe eggs, embryos, or species in which the developing embryo is nourished primarily by yolk stored within the egg. The yolk acts as a self-contained nutrient supply, providing lipids, proteins, and minerals during early development. In many animals, yolk production involves vitellogenin transport to the oocyte, with birds and many reptiles synthesizing yolk proteins in the liver before uptake into the developing egg.

In yolkcontaining eggs, the amount and distribution of yolk influence early development and the pattern of cleavage. Large yolk often leads to meroblastic cleavage, in which cell division occurs only in a portion of the cytoplasm or in a superficial layer. This pattern is typical of birds, most reptiles, monotremes, and many fishes. By contrast, eggs with relatively little yolk may undergo holoblastic cleavage, where cells divide throughout the entire egg.

Ecological and evolutionary implications include parental investment strategies and developmental timing. Yolk-rich eggs enable embryos to develop largely without immediate parental care after laying, which is common in oviparous species. In contrast, yolk-poor eggs or embryos in placentally nourished mammals depend more on maternal provisioning during gestation, or on post-laying care, depending on the lineage.

See also: yolk, vitellogenin, yolk sac.