1. How does amount of yolk influence cleavage in birds? How do we call this type of
Birds have telolethical yolk, which is there is a large amount of yolk towards one pole of the
egg and very less cytoplasm, which is known as blastodisc. Cleavage occurs on the side with
the very little cytoplasm on top of the yolk. The bird goes through discoidal meroblastic
cleavage. Cleavage produced a 5-6 cells layer thick with tight junctions.
2. How is hypoblast formed during gastrulation in birds?
Hypoblast is formed by the delamination and migration of the epiblast cells which form
3. Compare and contrast gastrulation in amphibians and birds. (think about different cell
movements due to the amount of yolk and the way blastocoel is formed)
In amphibians gastrulation occurs in animals that have very little or medium amount of yolk.
4. What is the relationship between primitive streak, primitive groove and Hensen’s node?
The primitive streak is the accumulation of cells in the middle layer between the epiblast and
the hypoblast. The epiblast also thickens at the posterior end. The primitive groove is when the
cells converge and depression occurs, which forms an opening allowing cells to pass to the
blastocoels. And the hansen’s node also known as the primitive knot forms at the anterior end
which consists of a funnel shaped end, by the cell can go through to the blastocoel.
5. What are the first cells which are passing through the primitive streak during
The first cells to pass through the primitive streak are the endodermal precursors
6. Compare and contrast formation of Nieuwkoop center in amphibians and birds. What is
the chicken equivalent to the Xenopus Spemann organizer? What is the chicken
equivalent to the amphibian dorsal blastopore lip? Explain your reasoning (in respect to
their role in gastrulation). You can use diagrams.
The Posterior Marginal Zone (BIRD) is similar to the Nieuwkoop centre (AMPHIBIAN), the
Nieuwkoop centre are the dorsal most vegetal cells of the blastula that induce organizer cells
(dorsal lip). The Nieuwkoop centre is formed when B-catenine on dorsal, which is determined
by gravity and where the sperm entry and Vg1which is excreted by the Vegetal overlap. The
PMZ is formed by the overlap of Vg1 and B-catenine. But in chickens the B-catenine is found
in the outer cells of the bastodisc and Vg1 due to gravity is at the presumptive posterior cells.
Hensen’s node (BIRD) =blastospore lip (AMPHIBIANS)
7. What are the characteristics of mammalian early cleavage?
(1) Very slow
(2) The orientation of blastomere cleavage varies relative to each other and secondary
cleavage goes through rotational cleavage.
(3) Division is not necessarily synchronic
(4) The genome is activated in early cleavage.
8. Define blastocyst. Draw blastocyst. Label ICM and trophoblast.