2. • The cleavage is the fractionating process which
splits zygote or fertilized egg into smaller cells
called blastomeres.
• These cells formed in typical doubling sequence
2,4,8,16,32,64 etc.
• The Geometrical regularity exist s only in early
developmental stages.
3. • Later on the process of cleavage becomes
entirely irregular.
• During early cleavage stages the blastomeres
form the structure of embryonic stage called
Morula.
• Later on Morula develops into the next
embryonic stage called Blastula.
4. Blastula
• As the cleavage proceeds ,the number of
blastomeres of the morula increases and
blastomeres undergo rearrangement.
• The cells adhere to each other and they arrange
themselves into true epithelium.
• This may be single cell thick (e.g.echinoderms)
or many cells thick (e.g.vertebrates)
5. • This epithelium is called blastoderm.
• The fluid filled space or cavity called blastocoel
appears in the centre of blastoderm.
• This hollow ,spherical, and unepithelial thick
embryonic stage is called blastula and the process
of its formation is called blastulation
8. A Dendraster blastula (micrograph
courtesy of George von Dassow,
Center for Cell Dynamics, Univ. of
Washington).
9.
10.
11. Types of Blastulae
• Coeloblastula
• Stereoblastula
• Periblastula or Superficial Blastula
• Discoblastula
• Blastocyst
12. Coeloblastula
• It is hollow sphere and blastocoel is filled with
mucopolysaccrides and blastoderm is of single
layer of cells.
• E.g. Echinoderms
13.
14. Stereoblastula
• It is a Solid blastula because there is no
blastocoel cavity.
• This type is formed due to spiral cleavage.
• E.g. Molluscs.
15.
16. Superficial Blastula
• It is also called periblastula.
• Periblastula is formed in insect eggs due to
superficial cleavge and there is no blastocoel in it.
• The nucei collect in the peripheral layer.
• Actually cavity is present but filled with yolk from
beginning of cleavage
17.
18. Discoblastula
• This type of blastula is found in yolky eggs of
fishes,reptiles and birds.
• It is called discoblastula because it appears at
the animal in the form of small multilayered
flat disc separated from yolk by narrow
subgerminal cavity.
19.
20. Blastocyst
• This type of blastula is formed in mammals by
regular cleavage
• The small cavity called blastcoel .
• The cluster of cells differentiates into two
distinct groups called trophoblast cells or
nutritive and formative cells
