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

Cleavage in Mammal

CLEAVAGE IN MAMMALS

• The egg of all marsupial and placental mammals are isolecithal and the cleavage is of holoblastic type and takes place within the zona pellucida of the ovum.
• The first two planes are vertical and third is horizontal.
• The darker blastomeres with slower cleavage are destined to become the embryo proper whereas the clear cells with rapid cleavage differentiate into auxillary tissue known as the Trophoblast.
• At about 16 cell stage, the future trophoblast cells begin to flatten against the zona pellucida and produce a kind of cellular capsule.
• By the time some thirty cells have formed, the definite hollow sac known as blastocyst. The cells destined to become embryo proper, constitute the inner cell mass attached to the trophoblast at one end.