Nucleus; 2) yolk (vitelline) granules; 3) follicular cells

Oligolecithal ovum is characterized by small amounts of yolk. In any oligolecithal ovum yolk is equally distributed within cytoplasm. Thus, all oligolecithal ova are isolecithal ova. A primitive vertebrate (such as Amphioxus, lancelet) leading an aquatic existence produces many small, yolk-poor eggs, which form free swimming larvae. Their ova are known as primary isolecithal ova. Through phylogenesis in eutherian mammals yolk has been secondarily reduced to the oligolecithal form, because their development occurs in a uterus. Thus, the human egg cell can be referred to oligolecithal, secondary isolecithal oocyte.

Polylecithal ovum is characterized by large amounts of yolk. Telolecithal ovum is characterized by the polarity of yolk allocation. One of the cell poles - the vegetative pole - is laden heavily with yolk, while another pole – the animal pole – is not so heavily yolked. The nucleus is placed at this animal pole. This polarity in yolk allocation may be pronounced more moderately – in mesolecithalova, or more sharply – in pronounced-telolecithal ova. Most amphibians have large, moderately heavily yolked (mesolecithal) eggs, which form free swimming larvae (tadpoils). In adaptation to a terrestrial existence, reptiles and birds produce few, large, yolk-rich (pronounced-telolecithal) eggs, with protective membranes and a shell, and have no larval stage.

CLEAVAGE

The zygote undergoes a rapid series of mitotic divisions called cleavage.

Cleavage is a rapid succession of consists of cell mitotic divisions following DNA synthesis but without cell grown: no increase in size, only increase in cell number.. These divisions result in a collection of small adherent cells called blastomeres. They do not grow in size but divide the zygote progressively into smaller cells.

The type of cleavage depends on the type of the egg cell in animal development. The amount and distribution of yolk in each cell determines the rate of cleavage and the relative size of the blastomeres.

Cleavage may be complete (holoblastic)or incomplete depending on extent of cytoplasmic subdivision (total or partial subdivision respectively), and to be equal or unequal depending on the relative sizes of the early blastomeres.

A series of cleavage divisions result in formation of blastula. Type of blastula depends on the cleavage type (Tabl.3, Fig.12).

Tabl.3 The principal types of cleavage and blastula

Fig.12. Blastula Types: a) celoblastula; b) periblastula (in Insects); c) amphiblastula;