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

CLEAVAGE IN AMPHIOXUS

• The egg of amphioxus is isolecithal and the cleavage is of holoblastic type. About one hour after fertilization, the egg is divided into 2 blastomeres.
• The plane of this first cleavage is passing through the egg axis from pole to pole.
• Next, the cleavage is in a vertical plane, but at right angles to the first plane.
• In the third series of division, the planes of separation are horizontal. By the 16 to 32-cell stage, the cells are crowded together into a compact mass within the zona pellucida. The embryo is now known as morula.
• Soon the balstomeres arrange themselves in a circular manner forming a blastula around a central space, the blastocoel. This marks the end of the cleavage period.