Summary of the main patterns of yolk accumulation and cleavage in animal embryology.


Summary of the main patterns of yolk accumulation and cleavage in animal embryology.

I. Holoblastic (complete) cleavage

II. Meroblastic (incomplete) cleavage

A. Isolecithal (sparse, evenly distributed yolk)

1. Radial cleavage (echinoderms, hemichordates, amphioxus)

2. Spiral cleavage (annelids, most mollusks, flatworms)

3. Bilateral cleavage (tunicates)

4. Rotational cleavage (placental mammals, nematodes, marsupials

B. Mesolecithal (moderate vegetal yolk disposition)

1. Displaced radial cleavage (amphibians, some fish [the lampreys, gars and bowfins)

A. Telolecithal (dense yolk throughout most of cell)

1. Bilateral cleavage (cephalopod molluscs)

2. Discoidal cleavage (some fish [the hagfishes, chondrichthyans and most teleosts], sauropsids [reptiles and birds], monotremes)

B. Centrolecithal (yolk in center of egg)

1. Superficial cleavage (most insects)


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