PATTERNS OF CLEAVAGE -
Patterns of cleavage depends upon amount of yolk & distribution of yolk.
These are following types -
1. RADIAL CLEAVAGE PATTERN -
In radial cleavage the cleavage furrow cut straight through the egg at right angle to another.]
It results blastomeres appear to be arranged radially. eg. Sponge & Coelenterata.
2. BIRADIAL CLEAVAGE PATTERN -
The cleavage furrow of first three division do not cut straight the axis of the egg.
It determine by radial symmetry in animal. eg. Ctenophora.
3. SPIRAL CLEAVAGE PATTERN -
In this the mitotic spindle of the third cleavage in the four blatomeres are laid down obliquely and are arranged in a sort spiral.
It result four blastomeres of upper tier do not lie over the corresponding blatomeres of lower tier. eg. Annelids, moluscs, nemerteans.
Spiral cleavage is of two types -
(i) Dextral or Dextrotrophic - Blastomere rotate clockwise (i.e. right hand spiral). eg. Mollusca.
(ii) Sinistral or Levotrophic - The blastomere rotate anticlockwise (i.e. left hand coiling). eg. Flat worm, Round worm & Annelids.
4. BILATERAL CLEAVAGE PATTERN -
In this two of the four blastomere are smaller than the other two and establish bilateral symmetry at the four cell stage. eg. Echinodermata & Chordata.