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Animal eggs are usually large and immobile cells, which contain nutrient reserves inside the embryo for development. These nutrients make up the calf. The quantity and location of the calf varies in different eggs. These two characteristics allow us to classify them into several types:

oligolithite (oligo = little; lecito = calf), homolecite or isolecite (homo or iso = equal) - has little evenly distributed calf and its segmentation is total or holoblastic (holo = whole; blast = germ) and equal, as it produces a morula with blastomers of approximately equal size; It is the egg of the protocordates (amphoxus and ascidian) and many marine invertebrates such as sponges, corals and starfish.

Representation of ovule oligolithite / alecith

alite (The = sem) - similar to oligoliths, but practically without calf. They are often classified as oligoliths or isoleciths. Your target is total or holoblastic (holo = whole; blast = germ) and equal, because it gives a morula with blastomers of approximately equal size.

heterolithite (hetero = different) - presents quantity of calf intermediate between eggs of oligoliths and teloleciths (hence the other names: mesolithite or mediolecith) and concentrated more on vegetable pole or vegetative that in animal pole (upper region); the targeting is total and uneven, for having less calf, the animal pole divides more rapidly and produces smaller and more numerous cells than those produced at the other pole; It is the egg of amphibians, various fish and some invertebrates (most mollusks, polychaetes and flatworms).

Total and equal segmentation of an oligolith (amphioxus) egg

Total and uneven segmentation of a heterolithite (amphibian) egg

telolecith (telo = tip) or megalithite (mega = big) - the nucleus and cytoplasm form a small drop on an enormous amount of calf (also called egg yolk, in this case); targeting is meroblastic (mere = part) or partial and discoid - because it occurs only at the animal pole and forms a small disc of cells (scar), stuck in the gem; It is the egg of reptiles, birds, various fish and some oviparous mollusks and mammals (platypus and echidna).

centrolecith - the calf occupies the central region of the cell and does not divide; the nucleus divides several times within the calf and migrates to the periphery, followed by the division of the cytoplasm; the segmentation is meroblastic and superficial; It is the egg of most arthropods (insects and others).


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