Classification of Eggs
(1) ON THE BASIS OF AMOUNT OF YOLK
(a) Alecithal eggs - Yolk is present in very less amount almost negligible. e.g. Eutheria mammals.
(b) Microlecithal or oligolecithal or Melolecithal egg- Yolk is present in less amount e.g. Metatheria, Protachordata, Echinodermata
(c) Mesolecithal Yolk is present in moderate amount. e.g. Amphibia, Petromyzon, Dipnoi-[lung fishes]
(d) Megalecithal or Polylecithal - Yolk is present in large amount. e.g. Insects, Reptiles, Birds, Prototheria (Monotremata) Egg laying mammals.
(2) ON THE BASIS OF DISTRIBUTION OF YOLK
(a) Isolecithal or Homolecithal - Yolk is distributed uniformally in eggs. e.g. Alecithal and microlecithal eggs.
(b) Moderately telolecithal - Yolk is concentrated in lower part of egg and cytoplasm in upper part e.g. Frog and other amphibia.
(c) Discoidal egg or Highly telolecithal- Yolk found in large amount and cytoplasm is found in the form of disc. e.g. Polylecithal eggs of reptilia, Birds, Prototheria.
(d) Centrolecithal - Yolk is concentrated in centre of egg and cytoplasm is found in the form of a thin layer surrounding yolk. (Peripheral cytoplasm) e.g. Insect.
Telolecithal and centrolecithal eggs are heterolecithal or anisolecithal eggs.
(3) ON THE BASIS OF SHELL
Two of types (a) Cleidoic eggs (b) Non-cleidoic egg
(a) Cleidoic egg - These eggs contain a thick and hard outermost shell. This hard shell is permeable for gases. Yolk, Salts and Water is present in large amount in cleidoic eggs. Cleidoic egg is a terrestrial adaptation. e.g. Birds & Reptiles, Prtotheria mammal and insects. (b) Non cleidoic egg - Egg membranes are soft in these eggs e.g. All viviparous animals and in oviparous animals which lays eggs in water.
Egg Envelops
All membranes (covering) which are present outside to plasma membrane of oocyte or ovum is known as egg membranes now instead on membrane envelop term is used. Envelop containing oocyte or ovum is called as egg
Egg envelope on the basis of origin are of three types.
(1) Primary egg envelops - These are secreted by egg.e.g. Vitelfine membrane
(Zona pellusida & Zona Reticulate)
(2) Secondary egg envelops - These are secreted by ovary. e.g.- Chorion on insects.
(3) Tertiary egg envelops - These are secreted by oviduct. e.g.- Jelly coat of frog, Albumin, shell membrane and shell of Hen.
Examples of Eggs
(1) EGG OF INSECT
These are megalecithal polylecithal & centrolecithal eggs.
The cytoplasm of insect egg is limited to periphery only whole
central place is filled by yolk.
Two types of egg envelops are present on egg of insects.
Inner - Vitelline membrane - Primary egg membrane.
Outer - Chorion - Secondary egg membrane. Chorion is thick, hard and ornamented. This ornamentation is taxonomically important in insects. Egg membrane is absent on a definite place, this place is known as micropyle. Micropyle is a place to enter for sperms in egg. Acrosome is absent in sperms of insects.
(2) EGG OF FROG
These are Mesolecithal and telolecithal egg.
Two types of egg membranes are present on frog egg.
(i) Inner - Vitelline membrane - Primary egg. membrane. It is secreted by egg cell.
(II) Outer - Jelly coat - Tertiary egg membrane (Secreted by oviduct).
All eggs are layed at a time. These eggs are attached together by jelly coat. This group is called as spawn.
Spawn formation in frog occurs after false copulation (amplexus).
Importance of Jelly coat
Air bubbles are present in jelly coat. These provide buyoancy.
Jelly coat protects egg from outer mechanical injury.
The jelly is tasteless hence it prevents the eggs from being eaten by water snails, aquatic insects, fishes and birds.
It prevents infection of bacteria and fungal spores.
The jelly reflects sun-rays, due to melanin it is also protected from UV rays.
The jelly is able to keep the eggs warm.
Like other eggs frog egg also have two poles :
(i) Animal pole - Cytoplasm and nucleus are found in this pole.
(ii) Vegetal pole - It mainly have yolk white yolk is filled in this pole. It is heavy so it is found in lower side. In animal pole in cytoplasm melanin pigments are also present. Melanin protect the egg from harmful radiation and helps in camouflage.
Sperm enters into egg through animal pole. The entry site of sperm in egg forms anterior part of embryo. Along with entrance of sperm determines the future polarity. The entrance path of sperm determine the plane of cleavage in egg. First cleavage of egg starts from right angle of the entrance path of sperm. From opposite direction of entry of the sperm, melanin granules move towards the sperm . So a clear region is formed between animal pole and vegetal pole. This clear region is known as "Grey crescent". it produces drosal lip of blastopore in future.
(3) EGG OF CHICK
These are megalecithal or polylecithal, discoidal and clidoic: Yolk is present in large amount and cytoplasm is found in the form of a disc. This disc is known as Germinal disc.
Beneath the germinal disc yolk is present. On the basis of colour and conent yolk
Is of 2 types Yellow yolk-It have more phospholipids White yolk – It is rich in protein.
Phospholipids are in less amount. Both types of yolks are arranged in alternative and concentric layers. Innermost and Outermost layer is of white yolkA stalk like structure of white yolk is present just beneath the germinal disc. This is known as latebra and upper part of latebra is known as Isthmus of pander. Primary envelop vitelline membrane is found around the egg.Remaining all envelops of chick egg are secreted by oviduct after fertilization. These are tertiary envelops. [Birds do not have copulatory organs but still they perform internal fertilization]. Egg of chick is filled with liquid albumin which is called as "White of egg". It is produced by lsthumus part of oviduct. Dense albumin on the sides of egg upto shell membrane are coiled chord like structures.These are called as Chalaza. It keeps the ovum upward.
Two shell membranes are present just beneath the shell. These are made up of keratin. A air pocket or air space is found between shell membranes in broad (wider) part of egg. The outermost part of egg is a shell made up by calcium carbonate [CaCO3). This shell is porous for exchange of gases.
The oviduct of birds is differentiated into three parts —
(i)Anterior part -- Magnum part—Fertilization of egg occurs in this part. After fertilization this part secretes liquid albumin arround the egg.
(ii) Middle part — Isthmus — Keratin shell membranes are secreted around the egg by this part. (iii) Posterior part — Nidamental part — This part secretes a shell of CaCO3 around the egg.
(4) EGG OF EUTHERIAN MAMMALS: Eggs are alecithal, non-clidoic and microscopic (0.1mm) in metatheria and eutheria. In prototheria egg are same as hen egg.
Two membranes or egg envelopes are present on egg-
(i) Inner envelop : Zona pellucida — Primary egg membrane produced by egg itself. In few mammals zona pellucida is linear and called as zona radiata.
(ii) Outer envelop : Corona radiata — formed by follicular cells of ovary. Follicular cells are attached with surface of egg through Hyaluronic acid.
Hyaluronidase enzyme is present in Mammalian sperm, which dissolves hyaluronic acid and the corona radiata cells are separated from egg surface. This helps in sperm entery.
Egg cytoplasm have very less or negligible amount of yolk.
On the basis of ovulation mammals are of two types
(i) Induced or Reflex ovulators — Animals in which ovulation is induced by copulation.
eg. All mammals which have a fix breeding period. The chances of fertilization increases to 100% in it e.g. Rabbit
Spontaneous ovulators Ovulation is not depended upon the copulation. Egg passes out in oviduct from ovary after fix period. The chances of fertilization are very low in these animals.
(1) ON THE BASIS OF AMOUNT OF YOLK
(a) Alecithal eggs - Yolk is present in very less amount almost negligible. e.g. Eutheria mammals.
(b) Microlecithal or oligolecithal or Melolecithal egg- Yolk is present in less amount e.g. Metatheria, Protachordata, Echinodermata
(c) Mesolecithal Yolk is present in moderate amount. e.g. Amphibia, Petromyzon, Dipnoi-[lung fishes]
(d) Megalecithal or Polylecithal - Yolk is present in large amount. e.g. Insects, Reptiles, Birds, Prototheria (Monotremata) Egg laying mammals.
(2) ON THE BASIS OF DISTRIBUTION OF YOLK
(a) Isolecithal or Homolecithal - Yolk is distributed uniformally in eggs. e.g. Alecithal and microlecithal eggs.
(b) Moderately telolecithal - Yolk is concentrated in lower part of egg and cytoplasm in upper part e.g. Frog and other amphibia.
(c) Discoidal egg or Highly telolecithal- Yolk found in large amount and cytoplasm is found in the form of disc. e.g. Polylecithal eggs of reptilia, Birds, Prototheria.
(d) Centrolecithal - Yolk is concentrated in centre of egg and cytoplasm is found in the form of a thin layer surrounding yolk. (Peripheral cytoplasm) e.g. Insect.
Telolecithal and centrolecithal eggs are heterolecithal or anisolecithal eggs.
(3) ON THE BASIS OF SHELL
Two of types (a) Cleidoic eggs (b) Non-cleidoic egg
(a) Cleidoic egg - These eggs contain a thick and hard outermost shell. This hard shell is permeable for gases. Yolk, Salts and Water is present in large amount in cleidoic eggs. Cleidoic egg is a terrestrial adaptation. e.g. Birds & Reptiles, Prtotheria mammal and insects. (b) Non cleidoic egg - Egg membranes are soft in these eggs e.g. All viviparous animals and in oviparous animals which lays eggs in water.
Egg Envelops
All membranes (covering) which are present outside to plasma membrane of oocyte or ovum is known as egg membranes now instead on membrane envelop term is used. Envelop containing oocyte or ovum is called as egg
Egg envelope on the basis of origin are of three types.
(1) Primary egg envelops - These are secreted by egg.e.g. Vitelfine membrane
(Zona pellusida & Zona Reticulate)
(2) Secondary egg envelops - These are secreted by ovary. e.g.- Chorion on insects.
(3) Tertiary egg envelops - These are secreted by oviduct. e.g.- Jelly coat of frog, Albumin, shell membrane and shell of Hen.
Examples of Eggs
(1) EGG OF INSECT
These are megalecithal polylecithal & centrolecithal eggs.
The cytoplasm of insect egg is limited to periphery only whole
central place is filled by yolk.
Two types of egg envelops are present on egg of insects.
Inner - Vitelline membrane - Primary egg membrane.
Outer - Chorion - Secondary egg membrane. Chorion is thick, hard and ornamented. This ornamentation is taxonomically important in insects. Egg membrane is absent on a definite place, this place is known as micropyle. Micropyle is a place to enter for sperms in egg. Acrosome is absent in sperms of insects.
(2) EGG OF FROG
These are Mesolecithal and telolecithal egg.
Two types of egg membranes are present on frog egg.
(i) Inner - Vitelline membrane - Primary egg. membrane. It is secreted by egg cell.
(II) Outer - Jelly coat - Tertiary egg membrane (Secreted by oviduct).
All eggs are layed at a time. These eggs are attached together by jelly coat. This group is called as spawn.
Spawn formation in frog occurs after false copulation (amplexus).
Importance of Jelly coat
Air bubbles are present in jelly coat. These provide buyoancy.
Jelly coat protects egg from outer mechanical injury.
The jelly is tasteless hence it prevents the eggs from being eaten by water snails, aquatic insects, fishes and birds.
It prevents infection of bacteria and fungal spores.
The jelly reflects sun-rays, due to melanin it is also protected from UV rays.
The jelly is able to keep the eggs warm.
Like other eggs frog egg also have two poles :
(i) Animal pole - Cytoplasm and nucleus are found in this pole.
(ii) Vegetal pole - It mainly have yolk white yolk is filled in this pole. It is heavy so it is found in lower side. In animal pole in cytoplasm melanin pigments are also present. Melanin protect the egg from harmful radiation and helps in camouflage.
Sperm enters into egg through animal pole. The entry site of sperm in egg forms anterior part of embryo. Along with entrance of sperm determines the future polarity. The entrance path of sperm determine the plane of cleavage in egg. First cleavage of egg starts from right angle of the entrance path of sperm. From opposite direction of entry of the sperm, melanin granules move towards the sperm . So a clear region is formed between animal pole and vegetal pole. This clear region is known as "Grey crescent". it produces drosal lip of blastopore in future.
(3) EGG OF CHICK
These are megalecithal or polylecithal, discoidal and clidoic: Yolk is present in large amount and cytoplasm is found in the form of a disc. This disc is known as Germinal disc.
Beneath the germinal disc yolk is present. On the basis of colour and conent yolk
Is of 2 types Yellow yolk-It have more phospholipids White yolk – It is rich in protein.
Phospholipids are in less amount. Both types of yolks are arranged in alternative and concentric layers. Innermost and Outermost layer is of white yolkA stalk like structure of white yolk is present just beneath the germinal disc. This is known as latebra and upper part of latebra is known as Isthmus of pander. Primary envelop vitelline membrane is found around the egg.Remaining all envelops of chick egg are secreted by oviduct after fertilization. These are tertiary envelops. [Birds do not have copulatory organs but still they perform internal fertilization]. Egg of chick is filled with liquid albumin which is called as "White of egg". It is produced by lsthumus part of oviduct. Dense albumin on the sides of egg upto shell membrane are coiled chord like structures.These are called as Chalaza. It keeps the ovum upward.
Two shell membranes are present just beneath the shell. These are made up of keratin. A air pocket or air space is found between shell membranes in broad (wider) part of egg. The outermost part of egg is a shell made up by calcium carbonate [CaCO3). This shell is porous for exchange of gases.
The oviduct of birds is differentiated into three parts —
(i)Anterior part -- Magnum part—Fertilization of egg occurs in this part. After fertilization this part secretes liquid albumin arround the egg.
(ii) Middle part — Isthmus — Keratin shell membranes are secreted around the egg by this part. (iii) Posterior part — Nidamental part — This part secretes a shell of CaCO3 around the egg.
(4) EGG OF EUTHERIAN MAMMALS: Eggs are alecithal, non-clidoic and microscopic (0.1mm) in metatheria and eutheria. In prototheria egg are same as hen egg.
Two membranes or egg envelopes are present on egg-
(i) Inner envelop : Zona pellucida — Primary egg membrane produced by egg itself. In few mammals zona pellucida is linear and called as zona radiata.
(ii) Outer envelop : Corona radiata — formed by follicular cells of ovary. Follicular cells are attached with surface of egg through Hyaluronic acid.
Hyaluronidase enzyme is present in Mammalian sperm, which dissolves hyaluronic acid and the corona radiata cells are separated from egg surface. This helps in sperm entery.
Egg cytoplasm have very less or negligible amount of yolk.
On the basis of ovulation mammals are of two types
(i) Induced or Reflex ovulators — Animals in which ovulation is induced by copulation.
eg. All mammals which have a fix breeding period. The chances of fertilization increases to 100% in it e.g. Rabbit
Spontaneous ovulators Ovulation is not depended upon the copulation. Egg passes out in oviduct from ovary after fix period. The chances of fertilization are very low in these animals.
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