Fertilisation is the process of fusion of two mature, haploid (23 chromosomes each) gamete cells, one from each parent, i.e., an Ovum and a spermatozoa. It marks the completion of the second meiotic division. 

Stages of fertilisation –

  1. Approximation of gametes
  2. Contact and fusion of gametes
  3. Results of fertilisation 

Fertilisation results in formation of undifferentiated, diploid (46 chromosomes) mononucleated single cell known as “zygote”.

The zygote undergoes a number of mitotic divisions, resulting in formation of “3-celled” [ known as Embryo ] stage, “4-celled” stage, and so on. The process of subdivision of cells is known as cleavage. The stages of subdivision are as follow-

  1. Stage of compaction – starts from 8 celled stage ( third division)
  2. Morula – 16 celled stage ( fourth division)
  3. Blastocyst – 32-64 cell stage (fifth and sixth division)

The process of cleavage lasts for 6 days, which occurs in fallopian tubes, from where it is transferred to the uterine cavity on around 4th day via ciliary beats of uterine epithelium and contraction of uterine tube musculature.

Age Stage Special Feature
0 daysFertilisation
1 day One celled stageZygote
2 daysFour celled stageEmbryo 
2-3 daysEight celled stageExhibit polarity
3-4 days16 celled stageMorula 
4-5 days32- 64 celled stageBlastocyst formation
6th dayHatching of blastocyst 
7th dayImplantation in uterine cavity

Layers of Embryo-

At the very early stage, embryo proper has three germ layers-

  1. Endoderm (inner)
  2. Mesoderm (middle)
  3. Ectoderm (outer)

All tissues of the body are derived from these germ layers. Further differentiation in these germ layers occurs as-

  1. Amniotic cavity formation
  2. Formation of primary yolk sac
  3. Formation of extraembryonic mesoderm
  4. Formation of extraembryonic coelom
  5. Connecting stalk formation
  6. Formation of chorion and amnion
  7. Formation of secondary yolk sac
  8. Circular embryonic disc
  9. Formation of prochondal plate
  10. Formation of primitive streak
  11. Formation of intraembryonic mesoderm
  12. Extensions of intraembryonic mesoderm
Age Stage 
8th day Bilaminar disc formation
14 daysProchondral plate and primitive streak is seen
16 daysDisc is now three layered
Allantoic diverticulum starts appearing

Further development of Embryonic disc

During the 2nd and 3rd week of development, the embryonic disc changes in shape from circular to oval and then finally pear shaped.

The cells of epiblast at caudal end of bilaminar embryonic disc are pluripotent, and forms following structures-

  1. Definitive endoderm
  2. Intraembryonic mesoderm
  3. Definitive ectoderm
  4. Notochord
  5. Primordial germ cells

Formation of notochord

Notochord is a midline structure, which develops between primitive streak and prochrondal plate. The formation of notochord occurs through following stages-

  1. Formation of primitive knot
  2. Appearance of depression in centre of primitive knot, known as blastopore
  3. Multiplication of cells of primitive knot and formation of notochordal process
  4. The cavity of the blastopore extends and covers the whole notochordal process and results in formation of a notochordal canal.
  5. Later this canal wall becomes flattened and a notochordal plate is formed.
  6. Proliferation of the cells of the notochordal canal results in formation of solid rod of cells, known as definitive notochord.

NOTE- as the embryo enlarges, the notochord also enlarges and lies in mid line and later the space is occupied by the vertebral column. However, notochord do not participate in vertebral column formation, but part of it is present in each intervertebral discs as nucleus pulposus.

Formation and fate of Neural Tube-

Neural tube is formed from an ectoderm lying over the notochord, and extends from the prochondral plate to the primitive knot. Soon it is divisible into cranial and caudal parts and gives rise to the brain and spinal cord.


The number of changes in shape of embryo and inner structures occurs due to rapid growth of cells in the central part of the embryonic disc and growth of somites.

  1. Flat pear shape to cylindrical shaped embryo.
  2. Head and tail end starts building over the amniotic cavity, however they still lie close to each other. These bulging gives rise to head fold, tail fold and lateral folds.
  3. With the formation of head and tail fold, part of the yolk sac becomes enclosed within the embryo, giving rise to formation of primitive gut. The primitive gut is divisible as foregut, hindgut and midgut, based on their relation with head, caudal or intervening part.
  4. Gradually, communication with the yolk sac became narrower, smaller and now known as the definitive yolk sac. It is connected to the gut cia channel, known as vitellointestinal duct. Later, it forms an umbilical opening and connects it to the placenta.
  5. Umbilical opening- comprises of- vitellointestinal duct, mesoderm of connecting stalk ( later converts to Wharton’s jelly ), blood vessels [connecting embryo and placenta], a part of embryonic coelom.

Age Stage 
15 daysAppearance of primitive streak and definitive yolk sac
17 daysNotochordal process appears, heart tube is seen
19 daysConnecting stalk starts distinguishing, intraembryonic mesoderm formed
21 daysNeural groove is seen, head fold starts forming
3rd weekUmbilical vessels developed
Blood and vessel forming cells appear
Heart tube and pericardium formed
Formation of external genitalia begins
Neural tube begins to form


4th week

DaysStructure formed  
4th weekTongue start forming
Subdivisions of Heart tube visibleHeart begins to beatAortic arch begins to establish in the cranial and caudal end.Veins starts forming.
Mesonephric tubules starts forming
Urorectal septum starts forming
Appearance of optic sulcus and Otic placode
22Optic vesicle comes in contact with surface ectoderm
Appearance of 1st and 2nd pharyngeal arch
23Neural tube closes
Neural folds begin to fuse.Primordia of sensory ganglia formed.(spinal and cranial)
26Forelimb bud appears
28Hindlimb bud appears
Frontonasal, maxillary & mandibular process identifiable
Lens and nasal placode appear

5th week

DaysStructure formed  
5th weekFour pharyngeal arches are seen .Thyroid and thymus start forming.
Limbs become paddle shaped
Nasal pits established
Hypobranchial prominence become prominent
Stomach rotates and dilates.Intestinal loops begin to form.Cecal bud identifiable.
Spiral septum formed.Aortic arches formed.Lymphatic sac formedThe cardinal, umbilical and vitelline veins formed.Conduction system of heart formed.
Metanephrons formed
Formation of brain vesicle.Sympathetic ganglia formed.Cerebral hemisphere begins forming.
Adrenal glands begins to develop.
Eye primordium surrounded by loose mesenchyme.
Tubotympanic recess develops.

6th week

DaysStructure formed  
6th weekFormations of future digits (fingers) seen.Cartilaginous model of bone starts forming.
Tubercle for formation of ear pinna starts forming.
Palate process forms on each side from maxillary process.
Dental lamina of both upper and lower jaw established.
Stomach completes its rotation.Intestinal loops well formed.Urorectal septum starts dividing into cloacaAppendix becomes clearly visible.
Lungs enter in pseudoglandular stage (2nd to 4th month of IUL)
Coronary circulation estabilising.AV valves and papillary muscles forming.
Mesonephron well formed.Cloacal membrane divides into urogenital and anal membrane.
Choroid fissure formed.Lens vesicle seen.
Auricles start forming.Cochlea and semicircular canal starts forming.

7th week

DaysStructure formed  
7th weekMammary line established.
Thyroid gland reaches its definite position.
Rotation of limbs occurs.
Eyelids are established.Maxillary process fuses with the medial nasal process.
Salivary glands starts developing.
Separation in cloaca completed.Intestinal loop herniates out of abdominal cavity.
Heart septa completely formed.
Urogenital sinus established.
Solid eye lens is formed.
Mesenchymal condensation for 3 ear ossicles appears.

8th week

DaysStructure formed  
8th weekMelanoblast appears.
Elbow and knee established.Fingers and toes are free.Primary centre of ossification starts appearing in many bones.
Eye shift from lateral to frontal position.Bucconasal membrane ruptures
Enamel organs are formed.
Intestinal loop rotates counterclockwise.
Cerebellum starts forming.
Rathke’s pouch loses its connection with oral cavity.
Cochlea and semicircular canal assume their definitive external form.

9th to 12th week (till end of 3 months)

DaysStructure formed  
9th weekAnal membrane breaks down.
10th weekPalatal process and nasal septum fuse with each other.
Enamel organs become cup shaped.
Corpus callosum formed.
Scala vestibuli and scala tympani appear.
11th weekEpidermal ridges appear.Collagen and elastic fibres appear.
12th week Cells of neural crest begin to migrate to the skin.Melanoblast invades epidermis.
Primary centre of ossification is seen in all long bones.
Head and tail foldings are completed.Herniated coils of the intestine return back to the abdominal cavity.
Urethral folds fuse with each other.Prostate begins to develop.
Definite kidney becomes functional.
Cerebellar cortex and purkinje cells are formed.
Urine foration begins
Male and female external genitalia visible through USG.

Till end of 6 months

DaysStructure formed  
4th monthSurface ectoderm becomes multi-layered.
Dermal papilla formedNails appear.
Lungs completed their pseudoglandular stage.
Dentate nucleus seen.Myelination of nerve fibre begins.
5th monthDevelopment of eccrine sweat glands, rudiments of lactiferous ducts.
Lungs enter the canalicular stage.
Vagina gets canalised.
Length increases considerably.
Foetal movement felt by mother.
Hair on eye and head seen
6th monthEnamel and dentate formed considerably.Formation of the tongue is almost complete.
Rapid eye movement begins
Lungs secrete surfactant. (sign of lung maturity)

Till end of 9 months

DaysStructure formed  
7th monthLungs complete the canalicular stage and enter the saccular stage.
External acoustic meatus canalised.
CNS able to control respiration, if born preterm
8th monthTestes reaches scrotum.
Body weight increases rapidly.
Just before birth Cementum formed
Sulci and gyri appear over the cerebral hemisphere.

After birth

DaysStructure formed  
Soon after birthPeriodontal ligaments formed before teeth eruption.
Lungs enter alveolar stage ( remain in this stage till 8 years of life)
Changes in foetal blood vessels-Contraction of thick muscle wall of umbilical arteries.Lumen of umbilical veins and ductus venosus is occluded.Ductus arteriosus occluded.Pulmonary vessels increase in size.
Over a period of timeOccluded vessels become fibrous tissue and form ligament.
Closure of frontal and occipital fontanelle. 

{REFERENCE- Inderbir Singh’s Human Embryology}

Suggested book for further read on the topic with Homoeopathic Therapeutics – 

  1.  A Concise Textbook Of Obstetrics And Neonatology With Homoeopathic Therapeutics by Dr Trupti M. Deorukhkar, Find this book here- 
  2. Gynaecology & Obstetric Therapeutics, by Shrikant Kulkarni, Find this book here- 
  3. Homoeopathic Medicines For Pregnancy & Childbirth, by Richard Moskowitz, Find this book here- 

About the author

Dr. Mansi Aggarwal

Dr. Mansi Aggarwal
(In-House Editor,The Homoeopathic Heritage )
Dr. Mansi Aggarwal completed her BHMS from Bakson Homoeopathic Medical College & Hospital, Greater Noida in 2021.
She has bagged certificates in various emerging fields in Healthcare, like:- Food and Nutrition; Psychology; Therapeutic lifestyle Management, etc.
She has experience of about 3 years in patient care and presently
practising as LIFESTYLE CONSULTANT & HOMOEOPATHIC FAMILY PHYSICIAN at Ojas: The Homoeopathic Healing Core.
She had been selected for conduct of her STSH research project by CCRH, on topic- "Usefulness of Individualised Homoeopathic Medicine in Treatment of Cases of Stress related Headache in Females of 18-35 years of age".