Download Cleavage and Blastula Formation in Starfish - BioRocks-2

Name ______________________________ Date ________________ Period ___ CN __
Cleavage and Blastula Formation in Starfish and Amphibians
Lab
I. Cleavage:
In order for a fertilized egg or zygote to become a multicellular organism, the
zygote must divide by mitosis. During early development, this process of division is
known as cleavage.
The fertilized egg of a starfish and many other invertebrates has very little yolk
and it experiences an even pattern of cleavage where the forming cells are uniform in
shape and size. This uniformity also extends to the fluid filled hollow cavity or
blastocoel. The blastocoel of the starfish is relatively large and nearly circular in shape.
However, in an amphibian and many other vertebrates the fertilized egg does not
experience a uniform or even cleavage. Differential concentrations of cytoplasm and
yolk (if present) will affect the cleavage process. The upper portion of the egg, usually
richest in cytoplasm, is known as the animal pole, and the lower portion of the egg
containing more yolk as the vegetal pole. The first plane of cleavage may pass all the
way through the zygote or through only a part of the zygote.
A second cleavage division typically occurs at a right angle to the first, producing
four cells. The third cleavage division cuts horizontally to form eight cells, four on the
top and four on the bottom. The cells produced during these cleavage divisions are
known as blastomeres. If the blastomeres in the top "tier" lie directly above those in the
bottom tier, then the pattern of cleavage is said to be radial, a pattern characteristic of
echinoderms and chordates.
Eggs that contain a large amount of yolk in one hemisphere cleave unequally.
The first two cleavages split the egg through the poles. The third cleavage separates the
yolkier (vegetal) pole from the upper, less yolky (animal) part. The four cells in the
animal hemisphere are much smaller than the four in the vegetal hemisphere. These
cellular characteristics are characteristic of many vertebrate embryos, like a frog embryo.
Procedure:
1. Use the starfish cleavage site listed below to assist you with your microscope
observations:
http://www.uoguelph.ca/zoology/devobio/210labs/cleavage1.html
2. Obtain a composite slide of starfish (Asterias) development. This slide will
have all stages of development represented, including unfertilized eggs,
fertilized eggs, and cleavage stages. Some of the unfertilized eggs contain a
germinal vesicle, a large swollen nucleus containing the nucleolus (appears as
a black dot). The germinal vesicle breaks down at the end of prophase I and
the egg appears as an opaque sphere during the remainder of maturation.
Locate an unfertilized egg with a germinal vesicle and one without. On the
"Starfish & Frog Development" sheet at the end of this lab draw these two
examples of an unfertilized egg. Label the nucleus and nucleolus.
Name ______________________________ Date ________________ Period ___ CN __
Cleavage and Blastula Formation in Starfish and Amphibians
Lab
3. Find two-cell, four-cell, and eight-cell embryos on your slide of Asterias
development. Notice that all of the cells in this embryo are the same size. On
your sheet of paper, draw what you observe.
II. Formation of the Blastula:
Repeated cleavages will result in formation of a hollow ball of cells called a
blastula. The cavity inside the blastula is called the blastocoel. Even as early as this
blastula stage, groups or layers of cells are already destined to become particular organs
or organ systems; these layers of cells are known as presumptive germ layers. The major
germ layers and their derivatives are listed below:
Germ Layer
Derivatives
Epidermal ectoderm
Skin.
Neural ectoderm
Brain, spinal cord, and neural crest cells.
Chordamesoderm
Notochord and spinal disks in some organisms; ganglia.
Mesoderm
Skeleton, circulatory system, excretory system, and parts of organs
belonging to other systems.
Endoderm
Gut and associated out pocketing.
Procedure:
1. Re-examine the composite slide of starfish development. Locate an early
blastula stage. The cells will be large enough to see and will appear as a dark
ring of cells surrounding a lighter center. Remember that these are whole
mounts and you are looking through a hollow sphere. Draw your observations
on the sheet of paper "Starfish & Frog Development." Label blastomeres and
blastocoel.
2. Locate a later blastula stage. The cells of the blastula are now so small that it
will be difficult to distinguish individual cells. Again, the outer rim appears to
be dark. At this time, the cells of the blastula are covered by cilia, which allow
the blastula to spin and move. Draw your observations. Label blastomeres
and blastocoel.
3. Observe a slide of cleavage in a frog embryo. Make a sketch of the embryo
on the “Starfish & Frog Development” sheet.
4. Observe a slide of a frog blastula. Make a sketch of the embryo on the
“Starfish & Frog Development” sheet.
Name ______________________________ Date ________________ Period ___ CN __
Cleavage and Blastula Formation in Starfish and Amphibians
Lab
Frog Cleavage
III.
Frog Blastula
Questions:
1. By which process does the fertilized egg divide?
2. Why is the fertilized egg not a uniform sphere?
3. Compare the animal pole and the vegetal pole of the egg.
4. Describe how the first cleavage division and the second cleavage division
are related to each other.
Name ______________________________ Date ________________ Period ___ CN __
Cleavage and Blastula Formation in Starfish and Amphibians
Lab
5. Explain how the third cleavage division relates to the first and second
cleavage divisions.
6. What are blastomeres?
7. Compare the cell sizes of the animal hemisphere with those of the vegetal
hemisphere during the eight (8)-cell stage of the embryo.
8. What causes the cells of these two (2) hemispheres to be different?
9. What is a germinal vesicle?
10. List the five (5) germ layers that exist in the blastula.
A.
B.
C.
D.
E.
Name ______________________________ Date ________________ Period ___ CN __
Cleavage and Blastula Formation in Starfish and Amphibians
Lab
11. When do cilia appear and how do the cilia function?
12. Compare the blastocoel of the starfish blastula and the frog blastula.
Name ______________________________ Date ________________ Period ___ CN __
Cleavage and Blastula Formation in Starfish and Amphibians
Lab
Starfish & Frog Development Sketches
Unfertilized Egg w/o
Germinal Vesicle
Two (2) Cell Stage
Early Blastula Stage
Unfertilized Egg w/
Germinal Vesicle
Four (4) Cell Stage
Eight (8) Cell Stage
Late Blastula Stage