Yolk

Definition

The nutrient-rich cytoplasmic material stored within an egg or oocyte that fuels embryonic development until the larva or juvenile can feed independently. In insects and other arthropods, yolk consists primarily of lipoproteins (vitellin), lipids, and glycogen deposited during oogenesis; its quantity and distribution pattern (concentrated, diffuse, or absent) correlate with reproductive strategy and larval ecology. Yolk reserves are mobilized through specialized cells (yolk sac or vitellophages) during embryogenesis.

Etymology

Middle English yolke, yelke, from Old English geolca, geoloca, related to geolu 'yellow', referring to the characteristic color of bird egg yolk.

Example

In lecithotrophic insects such as grasshoppers, a large yolk mass occupies most of the egg volume and sustains the embryo through direct development; in contrast, parasitoid wasps produce yolk-poor eggs that hatch quickly so larvae can feed immediately on host tissues.

Synonyms

• deutoplasm
• vitellus

Related Terms

• lecithotrophy
• matrotrophy
• Vitellogenesis
• Vitellin
• oocyte
• embryonic diapause
• yolk sac

Usage Notes

Distinguish lecithotrophic (yolk-dependent) from matrotrophic (maternal nutrient transfer) reproduction. Yolk content varies enormously across arthropods: holometabolous insects often have moderate yolk relative to egg size because larvae feed externally, while many arachnids and some insects with abbreviated development carry exceptionally large yolk reserves. The adjective 'lecithal' (from Greek lekithos 'yolk') appears in terms like macrolecithal (large-yolked) and microlecithal (small-yolked) eggs.