UNIT-2
TYPES OF EGGS
1. Eggs can be classified based on the amount of yolk they contain into three main
categories: microlecithal, mesolecithal, and macrolecithal. This classification is significant
in understanding the developmental processes of various organisms.
Microlecithal eggs contain a very small amount of yolk relative to the overall size of the egg.
These eggs are typically small and have a high cytoplasmic content.
Yolk Amount: Minimal; insufficient to sustain long-term development.
Examples: Found in organisms such as:
Amphioxus (lancelets)
Eutherian mammals (placental mammals)
Development: These eggs often undergo direct development, with embryos hatching
into relatively mature forms.
Mesolecithal eggs have a moderate amount of yolk, which is unevenly distributed within the
egg. The yolk is concentrated at one pole, known as the vegetal pole, while the other pole, the
animal pole, contains more cytoplasm.
Yolk Amount: Moderate; allows for longer developmental periods compared to
microlecithal eggs.
Examples: Common in:
Amphibians (e.g., frogs)
Some fishes (e.g., lampreys)
Development: These eggs typically exhibit a distinct larval stage before reaching
adulthood.
Macrolecithal eggs contain a large amount of yolk, which significantly influences the
development of the embryo. These eggs are usually larger and have a lower cytoplasmic ratio
compared to their yolk content.
Characteristics
Yolk Amount: Abundant; provides sufficient nutrients for extensive embryonic
development.
Examples: Found in:
Birds (e.g., chicken eggs)
Reptiles (e.g., turtles)
Some fish (e.g., bony fish)
Development: These eggs can support full development within the egg without
requiring an aquatic environment, often leading to more advanced embryos at
hatching.
1. Eggs can be classified based on the distribution of yolk into three main
categories: isolecithal, centrolecithal, and telolecithal. This classification helps in
understanding how different organisms utilize yolk for embryonic development.
Isolecithal/ microlecithal eggs have a small amount of yolk that is uniformly distributed
throughout the cytoplasm. This even distribution allows for equal access to nutrients for all
parts of the embryo.
Yolk Distribution: Uniform; yolk is scattered evenly throughout the egg. Examples:
Common in: Amphioxus (lancelets), Echinoderms (e.g., sea urchins), Some mammals
(e.g., humans)
Centrolecithal eggs have a large amount of yolk concentrated at the center of the egg, with a
thin layer of cytoplasm surrounding it. This arrangement allows for efficient nutrient storage
while maintaining a small area for cellular activities.
Characteristics
Yolk Distribution: Concentrated in the center; cytoplasm forms a thin layer around
the yolk. Examples: Found in: Insects (e.g., grasshoppers), Some coelenterates
Telolecithal eggs have a large amount of yolk that is unevenly distributed, primarily
concentrated at one pole (the vegetal pole), while the other pole (the animal pole) contains
less yolk. This distribution affects the embryo's development and orientation.
Subcategories of Telolecithal Eggs
1. Slightly Telolecithal
Contains a small quantity of yolk. Yolk is unevenly distributed, with a higher concentration
at the vegetal pole. Examples: Eggs of some fish species.
2. Moderately Telolecithal
Yolk Amount: Contains a moderate quantity of yolk. Yolk is concentrated in the vegetal
hemisphere, causing the nucleus to shift towards the animal hemisphere. Examples: Eggs of
amphibians (e.g., frogs).
3. Extremely Telolecithal
Contains a large amount of yolk, occupying most of the egg. The entire vegetal hemisphere
and a significant portion of the animal hemisphere are filled with yolk, displacing the
cytoplasm and nucleus towards the animal pole.Examples: Eggs of reptiles and birds (e.g.,
chicken eggs).
3. The classification of eggs into cleidoic and non-cleidoic types is based on the presence or
absence of a protective shell, which has significant implications for the reproductive
strategies of various species.
Cleidoic eggs, also known as amniotic eggs, are characterized by:
Presence of a Hard Shell: These eggs have a solid, often calcareous (calcium
carbonate-based) shell that provides protection against environmental hazards and
prevents desiccation. The shell is typically porous, allowing for gas exchange while
keeping the embryo safe.
Terrestrial Adaptation: Cleidoic eggs are laid in terrestrial environments, which
allows the embryos to develop fully outside of water. This adaptation is crucial for the
survival of many reptiles and birds, as it enables them to reproduce in a variety of
habitats.
Nutritional Support: The egg contains all the necessary nutrients for the developing
embryo, eliminating the need for an aquatic larval stage. This is particularly evident in
species like birds and reptiles, where the young hatch directly from the egg.
Examples of organisms that produce cleidoic eggs include:
Birds (e.g., chickens, ostriches)
Reptiles (e.g., turtles, snakes)
Some mammals (e.g., monotremes like the platypus)
Non-cleidoic eggs, on the other hand, are defined by:
Absence of a Hard Shell: These eggs do not have a hard protective covering. Instead,
they are often surrounded by soft membranes or gelatinous layers that offer limited
protection.
Aquatic Environment: Non-cleidoic eggs are typically laid in water, where they can
develop into larvae. This reproductive strategy ties these species to aquatic habitats
for successful reproduction.
Limited Nutritional Resources: Non-cleidoic eggs generally contain less yolk and
nutrients compared to cleidoic eggs, necessitating a larval stage for further
development.
Examples of organisms that produce non-cleidoic eggs include:
Fish (e.g., salmon)
Amphibians (e.g., frogs)
Some invertebrates (e.g., certain insects)
4. Determinate and Indeterminate
The development of embryos can be classified into two main types based on the fate of the
cells during the early stages: determinate cleavage and indeterminate cleavage. These
classifications are essential in understanding how different organisms develop and
differentiate during embryogenesis.
Determinate cleavage, also known as mosaic cleavage, is characterized by:
Early Cell Fate Specification: In this type of cleavage, the developmental fate of
each cell (blastomere) is determined very early in the embryonic development. Each
cell has a specific role and cannot develop into a complete organism if isolated.
Typical in Protostomes: This type of cleavage is commonly found in protostomes,
which include groups such as arthropods (e.g., insects) and mollusks (e.g., snails).
Eg.
Nematodes (e.g., Caenorhabditis elegans): In nematodes, determinate cleavage leads
to a fixed pattern of cell division where specific cells are destined to become certain
tissues.
Indeterminate cleavage, also referred to as regulative cleavage, features:
Flexible Cell Fate: In this type, the developmental fate of the cells is not
predetermined. Early blastomeres can still develop into a complete organism if
separated from the rest of the embryo.
Typical in Deuterostomes: Indeterminate cleavage is characteristic of deuterostomes,
which include vertebrates (e.g., mammals) and echinoderms (e.g., sea urchins).
Example
Mammals (e.g., Humans): In mammals, indeterminate cleavage allows for the
formation of identical twins, as cells can separate and still develop into fully
functional embryos.
Summary of Differences
Feature Determinate Cleavage Indeterminate Cleavage
Cell Fate Fixed early; specific roles Flexible; can develop into any cell type
Organism Examples Nematodes, arthropods Mammals, echinoderms
Feature Determinate Cleavage Indeterminate Cleavage
Developmental Removal of a cell leads to missing Removal does not affect overall
Outcome structures development
Egg membranes
Egg membranes play a crucial role in protecting and supporting the developing embryo
within the egg. They are classified into three main types: primary, secondary,
and tertiary membranes. Each type has distinct characteristics and functions, which vary
across different species.
Primary Egg Membranes
Primary membranes are the first layers that surround the egg and are typically formed by
the oocyte itself during its development. These membranes are essential for providing initial
protection and structural integrity to the egg.
Types of Primary Membranes
Vitelline Membrane: A thin, transparent layer found in many animals, including
insects, amphibians, and birds. It is composed of mucopolysaccharides and fibrous
proteins.
Zona Pellucida: Found in mammals, this is a glycoprotein layer that surrounds the
plasma membrane of the oocyte, playing a critical role in fertilization by facilitating
sperm binding.
Zona Radiata: Present in some fish and amphibians, this membrane has a radiated
appearance due to the arrangement of microvilli.
Chorion: Found in lower chordates like teleost fishes, this membrane is a product of
surface ooplasm.
Secondary Egg Membranes
Secondary membranes are formed by follicle cells surrounding the oocyte. These
membranes provide additional protection and support to the egg as it matures.
Types of Secondary Membranes
Chorion: A tough outer covering found in the eggs of insects, ascidians, and
cyclostomes. It often contains micropyles for sperm entry.
Corona Radiata: This layer surrounds the zona pellucida in mammalian eggs and
consists of radially arranged follicle cells.
Tertiary Egg Membranes
Tertiary membranes are secreted by cells of the oviduct as the egg moves through it. These
membranes provide further protection and nutritional support to the developing embryo.
Types of Tertiary Membranes
Albumen: Commonly known as egg white, it surrounds the vitelline membrane in
bird eggs. It consists of several layers with varying densities.
Shell Membrane: A double membrane surrounding the albumen in bird eggs,
providing additional structural support.
Calcareous Shell: The hard outer layer found in reptilian and avian eggs, composed
primarily of calcium carbonate.
Structure of typical egg (FROG)
Frog eggs are classified as mesolecithal due to their moderate amount of yolk, which is
unevenly distributed. The egg has distinct regions that contribute to its functionality during
fertilization and early development.
Key Components
1. Animal Pole
Description: The upper hemisphere of the egg, which is darker in color due to
the presence of melanin pigments.
Contents: Contains the nucleus and a higher concentration of cytoplasm. This
region is where sperm typically enters during fertilization.
2. Vegetal Pole
Description: The lower hemisphere of the egg, which appears lighter because
it is rich in yolk.
Contents: Primarily filled with yolk (vitellin), which serves as a nutrient
source for the developing embryo.
3. Yolk
Type: The yolk is stored in the form of granules and provides essential
nutrients for the developing embryo.
Distribution: Concentrated at the vegetal pole, contributing to the unequal
cleavage pattern during early development.
4. Cortex (Egg Cortex)
Description: A jelly-like layer just beneath the plasma membrane that contains
cortical granules.
Function: Plays a role in establishing polarity, bilateral symmetry, and overall
organization of the egg. It also helps protect against mechanical injury and
infection.
5. Cortical Granules
Description: Membrane-bound spherical bodies located in the cortex
containing mucopolysaccharides.
Function: These granules are crucial for preventing polyspermy (the entry of
multiple sperm into one egg) after fertilization.
6. Vitelline Membrane
Description: The inner membrane surrounding the yolk, secreted by the egg
cell.
Function: Acts as a barrier to protect the contents of the egg and plays a role
in sperm recognition during fertilization.
7. Jelly Coat
Description: An outer layer that surrounds the egg, secreted by the oviduct.
Function: Provides buoyancy, protection from predators, and prevents
bacterial infections. It also reflects sunlight due to its melanin content,
protecting against UV radiation.
8. Perivitelline Space
Description: The fluid-filled space between the vitelline membrane and the
surface of the egg.
Function: Allows for free movement of the fertilized egg and facilitates
nutrient exchange.
