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.