Lecture 12: Morphogenesis 2 - Cleavage and Formation of Blastula

Question 1

What is radial cleavage? What organism is an example of radial cleavage?

Answer 1

the division occurs at right angle to the egg surface and the first few cleavages produce tiers of blastomeres that sit directly over each other. example of sea urchins

Question 2

What is spiral cleavage? What organism is an example of spiral cleavage?

Answer 2

successive divisions are at planes slight angles to each other, producing spiral arrangement of cells. example of mollusks and annelids

Question 3

Why does the amount of yolk affect the pattern of cleavage?

Answer 3

In yolk eggs undergoing otherwise equal cleavage, a cleavage furrow develops in the least yolky region and gradually spreads across the egg, but its progress is slowed or even halted by the presence of yolk

Question 4

What is the Hetwig Rule?

Answer 4

a cell orients the mitotic spindle along the longer axis of the cell before cell division and cleavage then occurs perpendicular to this axis

Question 5

What is symmetric division?

Answer 5

two cells have the same contents and same developmental potential; mitotic spindle is positioned centrally and each arm is of equal length

Question 6

What is asymmetric division?

Answer 6

two cells have different content and orient different fate; mitotic spindle moves away form the centre of the cell and arm is extended