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EMBRYOLOGY / DEVELOPMENTAL BIOLOGY
▢ Implantation:
❖ Timing: After 32-celled stage
▢ Introduction:
Table 1: Basic Terms
Term | Meaning |
|---|---|
Embryology / Developmental biology | Study of development of organism |
Scope | Zygote → death |
Embryogenesis / Ontogenesis | Entire process of development |
Blastogenesis | Development from asexual reproductive body |
Embryogenesis / strict sense | Development from fertilized egg / zygote |
Teratology | Study of abnormal embryonic development |
❖ Historical Figures:
Table 1: Embryology Pioneers
Title | Scientist |
|---|---|
Founder of embryology | Aristotle |
Father of modern embryology | Karl Ernst von Baer |
▢ Reproductive Types:
Table 1: Types of Development
Type | Meaning | Examples |
|---|---|---|
Oviparous | Egg-laying; external embryonic development | Insects, fishes, amphibians, reptiles, birds |
Ovoviviparous | Egg develops inside maternal body; no maternal nourishment | — |
Viviparous | Embryo develops in uterine wall until birth | Eutherian mammals, some elasmobranch fishes, reptiles |
Parthenogenesis | Virgin birth; development without fertilization | Lacerta saxicola / Russian lizard |
▢ General Phases of Development:
Table 1: Developmental Sequence
Phase | Main event |
|---|---|
Gamete formation | Sperm + ovum formation |
Fertilization | Male gamete + female gamete fusion |
Cleavage | Rapid mitotic divisions without growth |
Blastulation | Blastula / blastocyst formation |
Gastrulation | 3 germ layers formation |
Organogenesis | Organs develop from germ layers |
▢ Eggs:
❖ Chalazae: Suspensory ligaments of yolk in bird egg
❖ Classification Based on Yolk Amount:
Table 1: Egg Types by Yolk Amount
Type | Yolk amount | Examples / Note |
|---|---|---|
Microlecithal / Alecithal | Small / negligible yolk | Starfish, Amphioxus, eutherian mammals; human egg = alecithal |
Mesolecithal | Moderate yolk | Amphibians e.g. frog |
Macrolecithal | Large yolk | Arthropods, fishes, reptiles, birds, prototherians |
❖ Classification Based on Yolk Distribution:
Table 1: Egg Types by Yolk Distribution
Type | Yolk distribution | Examples |
|---|---|---|
Isolecithal | Uniform yolk distribution | Protochordates, echinoderms |
Telolecithal | Yolk concentrated at vegetal pole | Frog, reptiles, aves, fishes |
Centrolecithal | Yolk concentrated centrally | Arthropods |
❖ Classification Based on Shell:
Table 1: Egg Types by Shell
Type | Feature | Note |
|---|---|---|
Acleidoic | No shell | Unprotected egg |
Cleidoic | Insulated egg → albumen + membranes + shell | Shell ≈ 94% calcium carbonate |
❖ Egg Membranes:
Table 1: Egg Membranes
Type | Formation | Examples |
|---|---|---|
Primary membrane | Secreted by ovum | Jelly envelope in echinoderms; vitelline membrane in molluscs, amphibians, birds; zona pellucida in mammals |
Secondary membrane | Formed by ovarian follicle cells | Chorion in insects |
Tertiary membrane | Formed by maternal genital tract | Frog → albuminous jelly layers; birds/reptiles → albumen + shell |
▢ Sperm:
Table 1: Sperm Structure
Part | Main components |
|---|---|
Head | Nucleus + acrosome |
Neck | Centrioles |
Middle piece | Mitochondrial region |
Tail / main piece | Axial filament + cytoplasm |
Tail / end piece | Naked axial filament |
❖ Special Case: Amoeboid sperm → Ascaris
▢ Fertilization:
❖ Definition: Fusion of male + female gametes
❖ Types:
Table 1: Types of Fertilization
Type | Site / Feature | Example |
|---|---|---|
External fertilization | Outside body; usually in water | Frog |
Internal fertilization | Inside female reproductive tract | Humans, ovoviviparous animals, viviparous animals |
❖ Biochemical Interaction:
Table 1: Fertilizin–Antifertilizin Reaction
Molecule | Source | Nature / Role |
|---|---|---|
Fertilizin | Egg surface | Glycoprotein |
Antifertilizin | Sperm | Acidic protein |
Reaction | Egg + sperm recognition | Species-specific binding |
❖ Key Events:
- Sperm penetration by acrosomal enzymes
- Fertilization cone formation
- Fertilization membrane formation → blocks polyspermy
- Completion of meiosis II in vertebrates
- Amphimixis → male + female pronuclei fusion
- Diploid zygote formation
❖ Amphimixis: Fusion of male pronucleus + female pronucleus → diploid zygote
▢ Cleavage:
❖ Characteristics:
- •Rapid mitotic divisions
- •No growth between divisions
- •Blastomere size decreases
- •High metabolic rate
❖ Patterns:
Table 1: Cleavage Patterns
Pattern | Arrangement | Example |
|---|---|---|
Radial cleavage | Blastomeres arranged radially | Echinoderms |
Spiral cleavage | Spiral blastomere arrangement | Annelids |
Bilateral cleavage | Right-left symmetry | Mammals |
❖ Types:
Table 1: Types of Cleavage
Type | Subtype | Egg type | Example |
|---|---|---|---|
Holoblastic | Equal | Microlecithal eggs | Humans |
Holoblastic | Unequal | Mesolecithal eggs | Frog |
Meroblastic | Discoidal | Telolecithal eggs | Birds |
Meroblastic | Superficial | Centrolecithal eggs | Insects |
❖ Stages:
Table 1: Cleavage Stages
Stage | Feature |
|---|---|
Morula | Solid ball of cells; mulberry-like |
Blastula | Blastocoel-containing stage |
Blastocyst | Mammalian blastula; trophoderm present |
❖ Blastula Types:
Table 1: Blastula Types
Type | Example |
|---|---|
Coeloblastula | Amphioxus |
Stereoblastula | Molluscs |
Discoblastula | Birds |
Superficial blastula | Insects |
▢ Gastrulation:
❖ Purpose: Formation of 3 germ layers → ectoderm, mesoderm, endoderm
❖ Processes:
Table 1: Gastrulation Movements
Process | Meaning |
|---|---|
Epiboly | Expansion / overgrowth of ectodermal cells |
Emboly | Cell migration inward |
Invagination | Infolding of cell sheet |
Ingression | Individual cell migration inward |
❖ Structures Formed:
- •Archenteron → primitive gut
- •Blastopore → absent in amniotes
❖ Blastopore Fate:
Table 1: Protostome vs Deuterostome
Group | Blastopore fate | Example |
|---|---|---|
Protostomes | Blastopore → mouth | Annelids |
Deuterostomes | Blastopore → anus | Humans |
▢ Germ Layer Derivatives:
Table 1: Germ Layer Derivatives
Germ layer | Derivatives |
|---|---|
Ectoderm | Nervous system, epidermis |
Mesoderm | Muscles, skeleton |
Endoderm | Digestive tract lining |
All 3 layers | Tympanic membrane, epithelial tissues, digestive system components |
▢ High-Yield Recall:
Table 1: Embryology One-Liners
Fact | Answer |
|---|---|
Study of development | Embryology / developmental biology |
Founder of embryology | Aristotle |
Father of modern embryology | Karl Ernst von Baer |
Abnormal embryonic development study | Teratology |
Virgin birth | Parthenogenesis |
Human egg | Alecithal |
Frog egg | Mesolecithal + telolecithal |
Bird egg | Macrolecithal + telolecithal + cleidoic |
Arthropod egg | Centrolecithal |
Bird egg yolk ligament | Chalazae |
Mammalian primary egg membrane | Zona pellucida |
Fertilizin source | Egg surface |
Antifertilizin source | Sperm |
Pronuclei fusion | Amphimixis |
Cleavage | Rapid mitosis without growth |
Morula | Solid mulberry-like cell ball |
Mammalian blastula | Blastocyst |
Gastrulation | 3 germ layer formation |
Primitive gut | Archenteron |
Protostome blastopore | Mouth |
Deuterostome blastopore | Anus |
Nervous system origin | Ectoderm |
Muscle + skeleton origin | Mesoderm |
Digestive tract lining origin | Endoderm |
Q1.
The origin of frog's brain is
📅MOE 2060
Q2.
Archenteron in frog's embryo is developed due to
📅MOE 2069
Q3.
Type of cleavage in frog's zygote is
📅MOE 2068
Q4.
Vegetal hemisphere of frog's egg consists of
📅MOE 2068
Q5.
Development of frog is completed in which correct sequence?
📅KU 2011
Q6.
Blastopore in vertebrates develops as
📅IOM 2010
Q7.
Skeleton and muscles both develop from
📅IOM 2050•IOM 2010•MOE 2008
Q8.
Metamorphosis in frog is regulated by secretion of
📅IOM 2010
Q9.
In frog, coelom is derived from
📅MOE 2010
Q10.
During gastrula, frog's embryo is differentiated into
📅IOM 2008•KU 2008
Q11.
Morula is composed of
📅MOE 2069
Q12.
Yolk plug is seen in
📅MOE 2008•IOM 1991•IOM 1997
Q13.
The cavity that appears during embryogenesis of frog is called
📅MOE 2064
Q14.
The number of fetal membranes in man is
📅MOE 2063
Q15.
In frog, jelly around egg is deposited
📅BPKIHS 2005
Q16.
Which of the following is the outer cell layer of an early embryo?
📅MOE 2067
Q17.
Which of the following is true about mesoderm?
📅MOE 2002
Q18.
If an unfertilized ovum of frog is pricked with a micro needle, it will
📅MOE
Q19.
Blastopore is found in
📅IOM 1998
Q20.
Germ layers are seen in
📅BP 1997
Q21.
Metamorphosis in frog can be accelerated by
📅BPKIHS 1995
Q22.
Blastopore is present in the stage
📅BP 1994
Q23.
Human egg is
📅BPKIHS 2014
Q24.
Evenly distributed yolk is called
📅IOM 2013
Q25.
Blastopore in vertebrates develops into
📅IOM 2013
Q26.
Metamorphosis in frog's tadpole is delayed by
📅BPKIHS 2013
Q27.
Eggs of insects are
📅IOM
Q28.
At which stage are the germ layers in frog's embryo developed?
📅MOE 2013
Q29.
Archenteron in frog's embryo is developed due to
📅MOE 2012
Q30.
Which enzyme helps in fertilization in frog?
📅KU 2014
Q31.
Fertilizin is a chemical substance produced by
📅KU 2011
Q32.
Cleavage is
📅KU 2011