· Embryology or Developmental Biology refers to the series of changes from a zygote to a complete adult.
Some Important Terminology to Remember
· Blastogenesis: It is a process of developing an individual from an asexual reproductive body.· Embryogenesis: The development of the new individual from fertilized egg or zygote.
· Paedogamy: It is the sexual reproduction of animals in larval conditions.
· Oviparous: They are egg Laying animals.
· Their embryonic development occurs outside the body. Examples: insects, fish, amphibians, reptiles, birds etc.
· Ovoviviparous: Animals lay eggs inside their body and its development also takes place inside the maternal body but is not supplied by maternal nourishment. Example: Protherian mammals.
· Viviparous: Animal which gives direct birth to the young ones so developing embryo derives nutrients from the maternal body. Example: Eutherian mammals.
· Viviparous: Animal which gives direct birth to the young ones so developing embryo derives nutrients from the maternal body. Example: Eutherian mammals.
· Teratology: It is the study of abnormal embryonic development of animals.
· Apomixis: It is the substitution of sexual reproduction in plants by a form of reproduction that does not involve meiosis and syngamy.
· Aristotle is the founder of the science of Embryology.
· The father of Modern Embryology is Karl Ernst von Baer.
· The father of Modern Embryology is Karl Ernst von Baer.
· He proposed the Germ Layers Theory.
· At first, human sperm was seen by Hamm and Leeuwenhoek in 1677 A.D.
· At first, human sperm was seen by Hamm and Leeuwenhoek in 1677 A.D.
Various Types of Animal Eggs
A) ON THE BASIS OF THE AMOUNT OF YOLKa. Microlecithal or Alecithal or Oligolecithal or Meiolecithal egg:
· This type of egg contains a small amount of yolk or without the yolk.
· Examples: Eggs of Marsupials & Eutherian mammals and starfish
b. Mesolecithal or Medialecithal egg:
· This egg contains a moderate amount of yolk.
· This egg contains a moderate amount of yolk.
· Example: eggs of Frogs, Toads
c. Megalecithal or Macrolecithal or Polylecithal egg:
· Large amount of yolk is present in this egg.
· Large amount of yolk is present in this egg.
· Examples: eggs of fishes, reptiles, birds, prototherian mammals and insects.
B) ON THE BASIS OF DISTRIBUTION OF YOLK
a. Homolecithal / Isolecithal:
· In this type of egg yolk is evenly distributed throughout the egg.
· Example: Microlecithal eggs
b. Telolecithal:
· In this type of egg, much of the yolk is concentrated towards the vegetal pole.
· Example: Mesolecithal and Megalecithal eggs
c. Centrolecithal:
· Yolk placed centrally in the centre of egg which is surrounded by a rim of cytoplasm.
· Example: Insects eggs.
Note:
· Cleidoic egg: These are the egg of reptiles and birds which are surrounded by calcareous shells for the diffusion of oxygen and carbon dioxide. About 94% of the shell is made up of calcium carbonate.
· Chalazae are the supplementary ligament of yolk in birds' eggs.
· Cleidoic egg: These are the egg of reptiles and birds which are surrounded by calcareous shells for the diffusion of oxygen and carbon dioxide. About 94% of the shell is made up of calcium carbonate.
· Chalazae are the supplementary ligament of yolk in birds' eggs.
Types of Egg Membranes
· They are the membranes that surround the egg and are formed during oogenesis.
· They are the membranes that surround the egg and are formed during oogenesis.
· They are of the following types:
a. Primary egg membrane:
· It is formed around the plasma membrane of the egg which is secreted by the ovum itself.
a. Primary egg membrane:
· It is formed around the plasma membrane of the egg which is secreted by the ovum itself.
· Example: vitelline membranes of Mollusca and amphibians birds, Zona pellucida of mammals, Zona radiata of sharks, some amphibians and reptiles.
b. Secondary membrane:
· It is formed around the egg by the follicle cells of the ovary.
· Example: chorion formed around the egg of insects.
c. Tertiary membrane:
· They are formed by the oviduct or other accessory part of the female reproductive system while the egg is passing from the ovary to the exterior.
· They are formed by the oviduct or other accessory part of the female reproductive system while the egg is passing from the ovary to the exterior.
· Example: the albumen, shell membrane and shell or reptiles and birds or three concentric albuminous jelly layers around the egg of the frog are secreted by the oviduct.
· Various Types of Cleavage
a) HOLOBLASTIC:
· Division is complete.
· It may be:
A. Equal: In microlecithal or isolecithal eggs
· Produces blastomeres of equal size
· Eg: Starfish, Amphioxus, marsupials and placental mammals.
A. Equal: In microlecithal or isolecithal eggs
· Produces blastomeres of equal size
· Eg: Starfish, Amphioxus, marsupials and placental mammals.
B. Unequal: In telolecithal or mesolecithal eggs.
· Produces unequal sized blastomeres, micromeres and macromeres.
· Eg: Amphibians.
· Produces unequal sized blastomeres, micromeres and macromeres.
· Eg: Amphibians.
b) MEROBLASTIC:
· Division is incomplete, i.e. occurring only in the cytoplasm, not in the yolk.
· Occurs in Macrolecithal eggs.
2 types:
i. Discoidal: in macrolecithal and telolecithal eggs.
· Eg: Reptiles, birds, monotreme mammals.
· Occurs in Macrolecithal eggs.
2 types:
i. Discoidal: in macrolecithal and telolecithal eggs.
· Eg: Reptiles, birds, monotreme mammals.
ii. Superficial: in macrolecithal and centrolecithal eggs.
· Eg: Most arthropods.
· Various Patterns of Cleavage:
· It is the fast mitotic division of the zygote in which the number of cells is increased but the size of the embryo remains constant.
· It is the fast mitotic division of the zygote in which the number of cells is increased but the size of the embryo remains constant.
· Its interphase is shorter than mitosis and has no growth of the embryo.
· So it is a successive mitotic division of egg to form the multicellular embryo.
· Cleavage was first observed by Swammerdam in frog egg.
The following types of cleavage are found in animals:
a) RADIAL: in this type, Blastomeres get arranged radially around the axis passing from animal pole to vegetal pole.
a) RADIAL: in this type, Blastomeres get arranged radially around the axis passing from animal pole to vegetal pole.
· Example- Sponges, Echinoderms, Coelenterates.
b) BILATERAL: Here Blastomeres get arranged in bilateral symmetry or blastomere of the right side is similar to those of left side.
· Example- Vertebrates
c) SPIRAL: In this case, Blastomeres get arranged spirally around the axis passing from animal pole to vegetal pole.
· Example- Annelida, Mollusca, Platyhelminthes.
TYPES OF CLEAVAGE
· Cleavage is determined by the amount of yolk and its distribution inside the egg.· Cleavages are of the following types:
1. Holoblastic cleavage:
1. Holoblastic cleavage:
· The division is complete so equal shape and size of blastomeres are formed from this cleavage.
· It is divided into two types:
A. Equal holoblastic cleavage
· It is divided into two types:
A. Equal holoblastic cleavage
· It produces blastomeres of equal size.
· It occurs in microlecithal and mesolecithal eggs.
· For example- starfish, amphioxus, marsupials, placental mammals.
B. Unequal holoblastic cleavage:
· It produces unequal size of blastomeres called micromeres and megameres.
· It produces unequal size of blastomeres called micromeres and megameres.
· For example- amphibians.
2. Meroblastic cleavage:
· In this division, cleavage furrow divides the small amount of active cytoplasm of animal pole or periphery of egg forming unequal blastomeres.
· In this division, cleavage furrow divides the small amount of active cytoplasm of animal pole or periphery of egg forming unequal blastomeres.
· Most of the yolky portion of the vegetal pole remains undivided.
· So it is partial or incomplete cleavage.
· It occurs macrolecithal or telolecithal eggs.
· It is divided into two types:
a. Discoidal cleavage
· It is divided into two types:
a. Discoidal cleavage
· In this case, cleavage remain restricted to the disc-shaped active cytoplasm of the animal pole.
· For example- eggs of reptiles, birds, monotremes, squids and fishes.
b. Superficial cleavage:
b. Superficial cleavage:
· At first nucleus located in the centre of the yolk divides repeatedly.
· Then daughter nuclei migrate through the yolk into the peripheral cytoplasm which then divides into many cells.
· For example- It is found in macrolecithal and centrolecithal eggs of arthropods.
Planes of Cleavage
· During cleavage, different cleavage furrows may divide the egg from different planes.
· During cleavage, different cleavage furrows may divide the egg from different planes.
· These are some cleavage planes found during cleavage:
1. Meriodinal plane: in this case, a cleavage furrow passes through the centre of the animal – the vegetal axis.
2. Vertical plane: here cleavage furrow passes from the animal pole to the vegetal pole but doesn't pass through the meridional axis.
3. Equatorial plane: Cleavage furrow divides the egg at the right angle to the main axis and halfway between the animal to the vegetal pole.
4. Latitudinal plane: It is similar to the equatorial but it passes through the cytoplasm on either side of the equatorial plane.
· This plane of division depends on the mitotic spindle of the dividing cell.
1. Meriodinal plane: in this case, a cleavage furrow passes through the centre of the animal – the vegetal axis.
2. Vertical plane: here cleavage furrow passes from the animal pole to the vegetal pole but doesn't pass through the meridional axis.
3. Equatorial plane: Cleavage furrow divides the egg at the right angle to the main axis and halfway between the animal to the vegetal pole.
4. Latitudinal plane: It is similar to the equatorial but it passes through the cytoplasm on either side of the equatorial plane.
· This plane of division depends on the mitotic spindle of the dividing cell.
Types of Blastula
· After cleavage solid ball like the stage is formed called a morula.
· After cleavage solid ball like the stage is formed called a morula.
· Then embryo changes into a blastula.
There are four types of blastula found in animals:
a) STEREOBLASTULA:
· In this type, blastocoel (cavity of blastula) is either very small or absent.
There are four types of blastula found in animals:
a) STEREOBLASTULA:
· In this type, blastocoel (cavity of blastula) is either very small or absent.
· So solid blastula is formed which composed of densely packed large-sized cells.
· Example- Coelenterates, Annelida and some Mollusca
b) COELOBLASTULA:
· Here, One to many layers of cells are arranged around a centrally or eccentrically placed blastocoel.
· Examples- Starfish, Frog
c) DISCOBLASTULA:
· In this case, a Few layers of cells over the yolk form a disc.
· The disc is called blastoderm and it is separated from the yolk by a space called subgerminal space.
· Examples- Fishes, Reptiles, Birds and egg-laying Mammals.
d) SUPERFICIAL BLASTULA:
· Yolk is surrounded by a layer of superficial single epithelial cells.
· There is no blastocoel.
· Example- Insects.
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