Biol 303: Lecture 2 Study Questions
1) Define the following terms: cleavage, blastomere, blastula, blastulation, microlecithal, mesolecithal,
megalecithal, centrolecithal, isolecithal, telolecithal, holoblastic cleavage, meroblastic cleavage, superficial
cleavage, discoidal cleavage, radial cleavage, spiral cleavage, bilateral cleavage, rotational cleavage,
determinate cleavage, indeterminate cleavage, blastocoele, direct development, indirect development,
protostomes, deuterostomes, epiboly, involution, invagination, ectoderm, mesoderm, endoderm,
delamination, cell migration, archenteron, primitive gut
Cleavage: period of rapid cell division of the egg cytoplasm that forms blastula. It’s a series of mitotic division
of the egg cytoplasm that form nucleated cells, known as the blastula.
Blastomere: A cell produced from cleavage through mitosis
Blastula: An early staged embryo that is made up of sphere shaped cells that surround an inner fluid-filled
cavity known as the blastocoel.
Blastulation: It is the stage in embryonic development in which the blastula is formed right after the morula
stage and then proceeds to gastrulation.
Microlecithal: very little amount of yolk in the haloblastic cells, formation of complete and separated cells.
Mesolecithal: moderate amount of yolk in the haloblastic cells.
Megalecithal: large amount of yolk
Centroecithal: yolk in the centre of the egg in the meroblastic cells, which are incomplete separated cells.
Isolecithal: uniform yolk distribution in haloblastic cells
Teloecithal: yolk at one end of the meroblastic cells, which are incomplete separated cells.
Holoblastic cleavage: Is the formation of complete and separated cells, this consist of very little yolk
Meroblastic cleavage: is the incomplete separation of cells because it contains a lot of yolk which prevent them
from going through cleavage.
Blastocoele: a fluid filled cavity in animal cells
Protostomes: develop the mouth first from the blastopore Biol 303: Lecture 2 Study Questions
Deuterostomes: develop anus first from the blastospore
Epiboly: movement of epithelial sheets that spread to enclose deeper layers of the embryo, by division, change
in shape, or by several layers intercalating.
Involution: inward movement of expanding outer layer over internal side of remaining external cells
Invagination: in-folding region of cells
Ectoderm: outer layer
Mesoderm: middle layer
Endoderm: inner layer
Delamination: splitting of one cellular sheet into two
Archenteron: when the first invaginate 1/4 to 1/2 the way into the blastocoel forming
Primitive gut: archenteron
2) Is protein synthesis essential during early cleavage? Is mRNA synthesis essential during early
Proteins and mRNA are essential during early cleavage. mRNA encodes cyclic B, which is responsible for
mitotic division in the S phase. Cyclic B is responsible for activating cyclin depended kinase, which is a protein.
3) Actinomycin D does not arrest early amphibian cleavage but blocks gastrulation. What sort of
conclusions can you draw from this observation?
4) What would you consider to be two most important things that an embryo accomplishes during the
process of cleavage and blastulation?
(1) The amount and the distribution of the yolk in the cytoplasm determines where cleavage will occur
and the formation of the blastomeres. The yolk free pole is the animal pole and the yolk rich pole is the vegetal
(2) The factors of the egg cytoplasm which influence the angle of the mitotic spindles.
5) How does the cell cycle which occurs during cleavage differ from the cell cycle of normal adult body
The cell cycle in the cleavage only has a S and M phase. Cyclin B synthesis allows the progression of M
(mitosis) and then the degradation of the cyclin B allows the cells to pass into the S (synthesis) phase. Where as
the cell cycle of the normal adult body cell goes through M (mitosis) which is then followed by the interphase
which consists of the G 1 S (synthesis), G 2hases.
6) Distinguish between complete (holoblastic) cleavage and incomplete (meroblastic) cleavage.
Holoblastic cleavage is the formation of complete and separate cells, which contain very little yolk and
meroblastic cleavage also known as incomplete is the incomplete separation of the cells as a result of too much
yolk. Biol 303: Lecture 2 Study Questions
7) Distinguish between determinate and indeterminate cleavage?
Indeterminate cleavage is when cells are separated from each other and have the potential to form into entire
organisms on their own.
Determinate cleavage is when cells are separated from each other, they will die. Neither of the cells have the
potential to develop into entire organisms on its own.
8) Comment briefly on the relative importance of DNA synthesis, RNA synthesis and protein synthesis
during cleavage. What do you think, what cellular structures are being most actively synthesized?
9) Why does the terrestrial environment present more difficulties for embryonic development than the
fresh water environment, and why does the fresh water environment in turn present more difficulties for
development than the marine environment?
Terrestrial environments are harsh and the amount of yolk is substantial unless carried internally. Fresh water
environments are less hospitable and the amount of yolk is moderate. Marine environment is stable/hospitable
and has small amount of yolk, leading to haloblastic cells.
10) What major embryonic and reproductive adaptations are seen in terrestrial and fresh water animals
for dealing with these environments and helping to ensure survival of the species?
The terrestrial animal eggs are enclosed in shell or carried internally or laid in moist environments. The Fresh
water animal eggs are retained by parent or get attached to the substrate.
11) What are the major developmental features which allow the metazoans to be classified into two
groups, the protostomes and the deuterostomes?
Bilateral symmetrical animals are divided into two groups, the protostomes and the