Equal division

Meiotic division of

spermatogonium produces 4 mature sperm

Sperm active


Unequal division

Meiotic division of Oogonium produces 1 oocyte and 3 polar bodies

Oocyte becomes metabolically dormant


2 processes

i) egg activation

Fusion of sperm and egg plasma membranes results in reactivating of egg metabolism

ii) nuclear fusion

Fusion of egg and sperm pronuclei

Diploid genetics

Fertilisation membrane produced

Ionic changes occur

Result in preventing more sperm entering the egg

Egg types

Yolk: Proteins, fats needed for energy and nutrition

Correlates with the time to hatching or attachment

Early development affected by amount of yolk

slows down divisions

4 types

  1. Isolecithal

iso = equal, lekithos = yolk

small amount of yolk evenly distributed in the egg

Echinoderms, amphioxus, mammals


telos = end, lekithos = yolk

Large amounts of yolk

Concentrated at lower end or vegetal pole

Upper or animal pole contains nucleus and cytoplasm

Molluscs, frogs, fish, reptiles, birds

c) Centrolecithal

Centro = middle, lekithos = yolk

Yolk concentrated in centre of egg

Cytoplasm present as thin layer on surface

Island of cytoplasm containing nucleus in centre

Unique to arthropods


After fertilisation, Zygote is produced

Series of rapid mitotic divisions occur: CLEAVAGE

Converts single celled zygote into multi celled BLASTULA

Distinctive developmental stage of animals

Defining feature of animal kingdom

Very rapid divisions eg 1-37,000 cells in 48hrs

Each new cell is called a blastomere

No growth of cells occurs

thus blastomeres get smaller

Cytoplasmic to nuclear ratio decreases

Two kinds of cleavage

1) Holoblastic

Entire zygote cleaves

Occurs in isolecithal and small yolked telolecithal eggs

2) Meroblastic

Incomplete cleavage

Most of yolk does not cleave

Cells form in the cytoplasm only

Holoblastic cleavage

1) Radial cleavage

eg Amphioxus

1st division from animal to vegetal pole (meridional)

2 cell stage

2nd division meridional but at 90 degrees to 1st

4 cell stage

3rd division is equatorial, right angle to meridionals

8 cell stage

All cells of approximately equal size

Divisions continue, alternating, forming 32, 64, 128 cells etc

These ball of cells called a blastula

Blastula can be bisected along any meridional and two mirror images result, i.e. Radial symmetry

Yolk used for energy, thus mass decreases, but overall size does not

Leads to hollow inside the blastula


Provides space for that facilitates cell movement in later development

In moderately yolked telolecithal eggs, eg frogs

Yolk slows divisions down

3rd starts before 2nd finishes

Produces micromeres and macromeres

Blastocoel displaced towards animal pole

Spiral cleavage

Most invertebrates, except echinoderms

1st cleavage is meridional

2nd cleavage is meridional

3rd division is unequal, cleavage planes are oblique

Mitotic spindles inclined

Four micromeres formed

Four macromeres formed

Each micromere lies over two macromeres

Meroblastic cleavage

Discoidal cleavage

Cephalopods, fish, birds, reptiles

Heavily yolked telolecithal

Superficial disc of cytoplasm cleaves

Meridional cleavage leads to single layered blastoderm

Equatorial cleavages divides this layer

Subgerminal cavity forms

Lower cells migrate into subgerminal space to form the blastocoelSuperficial cleavage

Centrolecithal eggs

Several nuclear divisions without cytoplasmic division

Nuclei migrate to cell surface

Forms a syncytium (many nucleus)

Cytoplasm division converts syncytium into a cellular blastoderm

Blastoderm surrounds a mass of uncleaved yolk