(1) Determinate / Mosaic eggs : Every part of fertilize egg has a definite fate, so that fate of every blastomere is determined from the beginning. It is found in invertebrates except echinoderms.
(2) Indeterminate / Regulative eggs : The fate of different parts of egg or its blastmeres is not predetermined. Example – Echinodermes, Vertebrates.
(c) On the basis of shell
(1) Cleidoic eggs : Eggs surrounded by a hard shell are know as cleidoic eggs. These eggs are found in those animals which have a terrestrial mode of life of which lay eggs on land. These eggs have more amount of yolk. These are adaptations to terrestrial mode of life. Shell prevents the egg from dessication. e.g. – Eggs of "Reptiles". "Birds". "Insects" and "Prototherians".
(2) Non - Cleidoic eggs : Eggs which are not surrounded by a hard shell are called Non-cleidoic eggs. These eggs are found in all oviperous animals which lay eggs in water and all viviperous animals. e.g. – All viviperous animals (Mammals) and all oviperous animals which lay eggs in water (Amphibians).
Structure of an oocyte : Oocyte is a large yolk containing cell. The nucleus is large and swollen and is termed as the "Germinal Vasicle". Oocyte is surrounded by membranes termed as the egg-membranes. Oocyte / Ovum alongwith the egg-membrane are termed as the egg. Egg = Ovum / Oocyte + Egg membrane.
Classification of egg – membranes : On the basis of origin, egg-membranes are of 3 types –
(1) Primary egg membrane : This membrane is secreted by the egg (ovum) itself. e.g. – Vitelline membrane, of human egg.
(2) Secondary egg membrane : This is found outside the primary egg membrane and is secreted by the ovary. e.g. Chorion of insect eggs, corona radiata and zona pellucida of human egg.
(3) Tertiary egg membrane : This present outside the primary and the secondary egg membrane. It is either secreted by the uterus or the oviduct. Egg jelly coat around frog's egg; albumen, shell membrane and shell of bird egg.
Functions of egg membranes
(1) To provide protection. (2) To check polyspermy.
(3) To provide buoyancy to the amphinian eggs. (4) To provide nutrition (Birds, Reptiles)
(5) To help in excretion (Allantois)

# (1) Insect egg : Eggs of insects are megalecithal or polylecithal in them yolk is present in the centre, so the eggs are also centrolecithal. Eggs of insects are cigar like. Along with plasma-membrane the egg has 2 egg-membranes.
(i) Vitelline membrane : This is a primary egg membrane and the egg itself secretes it around.
(ii) Chorion : This is a secondary egg membrane and is secreted by the ovary. In Insect's egg tertiary egg-membranes is absent. Chorion of insect's egg is ornamented i.e. there are specific markings on its egg membrane which are characters of Taxonomic importance. In the egg, a hole termed as micropyle is present which is the port of entry for sperms. Its cytoplasm is divided into 2 parts –
(a) Central (b) Peripheral cytoplasm
(a) Central cytoplasm : It is present in a very small amount in the centre of the egg. Egg nucleus is located in it.
(b) Peripheral cytoplasm : It is present in a very small amount along the periphery of the egg.
Yolk : In insect's egg yolk is present in a very large amount and this yolk is concentrated between the central and the Peripheral cytoplasm.

# (2) Frog's egg : Eggs of frog are Teloecithal and Mesolecithal. The egg has 2 egg membranes.
(i) Vitelline membrane : This is a primary membrane, secreted around by the egg itself.
(ii) Jelly – coat : This is a tertiary egg-membrane. It is secreted by the oviduct. Secondary egg-membrane are absent in these egg's. Internally, the egg is divided into 2 areas –
(a) Animal pole (b) Vegetal pole
- (a) Animal pole : This part has more amount of cytoplasm in it and the egg nucleus is also located in it. In this part melanin granules are found which prevent the egg from harmful radiations. Due to these melanin granules the frog's egg is partly white and partly black. This helps in Camouflage. Sperm always enters inside the egg through the animal pole. The part from where the sperm enters inside the frog's egg in future forms the ventral part of the embryo. As the sperm enters inside the egg. The part directly opposite to the entry point becomes a clear-zone due to the rapid movement of melanin granules. this clear-zone is termed as the Grey-Crescent. This part with Grey-Crescent forms the dorsal part of the embryo in future.
- (b) Vegetal pole : Here the yolk is concentrated in frog's egg, the part with cytoplasm in future forms the ectoderm. The Grey crescent part in future the Mesoderm and the part with yolk in future forms the endoderm.

# Jelly-coats of all the eggs of a frog absorb water and swell up, to form a cluster of eggs termed as Spawn. Jelly-coat has air-bubbles, due to which the eggs don't drown. Jelly-coat is bilter in taste and so the eggs are protected from the enemies.