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

CLEAVAGE

• The fertilized ovum enters on a series of cell division. The initial period, the development of a new multicellular individual is called the cleavage period. The process of cellular division without growth is called cleavage. During the period, the zygote split up into a number of smaller cells called the blastomeres. The mitotic division tends typically to follow the doubling sequence, 2, 4, 8, 16 etc. The total mass of living substance does not increase appreciably when the cleavage comes to an end.
• It is the active protoplasm of the egg that accomplishes the division and the inert stored yolk retards mitosis. In this way the relative amount of yolk and its even or uneven distribution throughout the egg, influence on cleavage. On this basis, the cleavage is classified as follows:
  • Total Holoblastic cleavage: Entire ovum divides
    • Equal: In isolecithal ova (evenly distributed yolk) blastomeres are of approximately equal size. Eg. Amphioxus, marsupial and placental mammals.
    • Unequal: In moderately telolecithal ova (uneven distribution of yolk) accumulated at the vegetal pole retards mitosis and fewer but larger blastomeres form there. Eg. Lower fishes and amphibians.  

• Partial Meroblastic cleavage: Protoplasmic region alone cleaves.
  • Discoidal: In higly telolecithal ova, mitosis is restricted to the animal pole. Eg. Higher fishes, reptiles, birds and monotreme mammals.
  • Superficial: In centrolecithal ova (the yolk substance is accumulated in the centre and protoplasmic mass located peripherally), the mitosis is restricted to the peripheral cytoplasmic investment. Eg. Arthropods.
• All the animals under the phylum chordate have certain similarities in their early development. It is therefore necessary to compare the rarely stages of development, in Amphioxus (a lower chordate), Chick and Mammal for gaining a broad understanding of the subject.