1. Gametogenesis
- Spermatogenesis
(formation of sperm in testes)
- Begins at
puberty
- Involves
mitosis → meiosis I → meiosis II → spermiogenesis
- Mnemonic: "Some Men Make Sperm" = Spermatogonia →
Primary → Secondary → Spermatids → Sperm
- Oogenesis
(formation of ova in ovaries)
- Begins in
fetal life
- Arrested
in prophase I until puberty
- Meiosis
II completes only at fertilization
2. Fertilization
- Occurs in ampulla
of fallopian tube
- Fusion of
sperm and ovum forms zygote
- Sex is
determined at fertilization (XX or XY)
3. Cleavage and Blastocyst
Formation
- Zygote →
2, 4, 8-cell stages → Morula (solid ball) → Blastocyst
- Inner cell
mass: becomes embryo
- Trophoblast:
becomes placenta
4. Implantation
- Occurs on 6th–7th
day post-fertilization
- Site:
Posterior wall of uterus
- Trophoblast
differentiates into:
- Cytotrophoblast: inner
layer
- Syncytiotrophoblast: outer
layer, invades endometrium
5. Bilaminar Germ Disc
- Appears in
2nd week
- Epiblast (dorsal)
+ Hypoblast (ventral)
Mnemonic: "Week 2 = 2 layers" — Epiblast +
Hypoblast
6. Trilaminar Germ Disc
(Gastrulation)
- Appears in
3rd week
- Formation
of 3 germ layers:
- Ectoderm (skin,
nervous system)
- Mesoderm
(muscles, bones, heart, kidneys)
- Endoderm (gut
lining, liver, pancreas)
Mnemonic: "E-M-E" = Ectoderm, Mesoderm,
Endoderm
7. Neurulation
- Formation
of neural tube from ectoderm
- Starts in 3rd
week, completes by end of 4th week
- Neural
crest cells give rise to PNS, melanocytes, adrenal medulla
Mnemonic: "Neural Crest = MAGIC":
- Melanocytes
- Adrenal
medulla
- Ganglia
(PNS)
- Iris
muscles
- Cranial
nerves
8. Somite Formation
- From paraxial
mesoderm
- Forms:
- Sclerotome
(vertebrae)
- Myotome
(muscles)
- Dermatome (skin
dermis)
Mnemonic: "SMD" = Sclerotome, Myotome,
Dermatome
9. Pharyngeal (Branchial) Arches
- Appear in
4th–5th week
- Each arch
contains artery, nerve, cartilage, and muscle
Mnemonic: "5-7-9-10" for cranial nerves of
arches 1–4:
- 1st: CN V
(Trigeminal)
- 2nd: CN
VII (Facial)
- 3rd: CN IX
(Glossopharyngeal)
- 4th &
6th: CN X (Vagus)
10. Development of Organs
- Heart: starts
beating by day 22
- Lungs: begin
development in 4th week
- Kidneys: develop
in stages (pronephros → mesonephros → metanephros)
11. Fetal Membranes
- Amnion: forms
amniotic fluid
- Chorion: outer
membrane
- Yolk sac: early
blood formation
- Allantois: involved
in early waste removal
12. Placenta
- Formed by:
- Fetal
part: chorion frondosum
- Maternal
part: decidua basalis
Mnemonic: "Fetus Frondosum, Mom Basalis"
13. Twins
- Dizygotic
(Fraternal): 2 eggs, 2 sperm → separate placenta & sacs
- Monozygotic
(Identical): 1 zygote splits → shared placenta, variable sacs based on timing
Mnemonic: "2 eggs = 2 everything (di-di); 1 egg =
mono"
