VAN 211: Veterinary Histology and Embryology (2+2)

CLEAVAGE IN CHICK

• The egg is large and highly telolecithal and the cleavage is of meroblastic type (partial and discoidal). The protoplasm is located as a cap over the huge yolk mass and is involved in the cleavage. The large yolk never divides.
• The first 2 planes of division are vertical furrows which cross at right angles. Succeeding furrows pass first radial and then in circumferential planes and the original disc of cytoplasm is transformed into a mosaic of separate cells. Following this stage, cleavage divisions take place in a horizontal plane to produce a certain amount of layering.
• The end result, after one day, is a discoidal plate of cells perched on a surface of the yolk and separated from it by a cleft. At the periphery, the cellular disc progressively gains new cells from a proliferating, syncytial margin that blends into the yolk. The cellular cap is termed as germinal disc or blastoderm. The space between the blastomeres and yolk mass is called the blastocoel. Cleavage thus produces a modified blastula corresponding to the blastula of the amphioxus.