Download Sea Urchin (Pluteus) Eggs: Cleavage and Gastrulation

LAB: Sea Urchin Eggs: Cleavage and Gastrulation
Introduction
The pattern of the early stages in the development of the embryo is very much the same in most animals. A
fertilized egg, or zygote, undergoes a series of rapid cell divisions called cleavage. In this process, the onecelled zygote divides into many smaller cells. Together, such cells constitute an embryo.
At first, the embryo appears spherical (round) without special organs or tissues. Later, as internal changes occur,
the shape of the embryo changes gradually. Cleavage is considered to be over when the rate of cell division
slows down and the cells begin to differentiate. That is, the cells, which at first are very similar, change in
structure and form tissues that have different functions.
At fertilization, the egg produces a new outer membrane called the fertilization membrane. Therefore, when
viewed under the microscope a fertilized egg will appear as a single cell with a double membrane (While an
unfertilized egg will possess only a single membrane, the plasma membrane). The major stages in cleavage are
the two-celled, four celled, eight-celled, and sixteen-celled stages followed by the morula (solid ball of cells,
appears knobby like a blackberry), blastula (hollow ball of cells), and the gastrula stage. The start of
gastrulation is marked by the pushing inward of cells that will form a long channel or primitive gut.
Materials
Stained slides of cleavage (starfish or sea urchin) – all stages
Compound Microscope
Lens paper
Procedure and Observations
1. Examine a slide of Sea Urchin (or Starfish) embryos under low power and locate a field of view in which
there are several specimens.
2. Moving the slide as needed, locate and draw an unfertilized egg, a zygote, embryo with consisting of two,
four, and eight cells (not all cells can be seen at this point), and a morula (around 32 cell stage).
Unfertilized Egg
Zygote
Two-celled Embryo
Four-celled Embryo
Eight-cell Embryo
Morula
a. How does the size of the blastula compare to the size of the morula, how can you explain this?
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b. How come only one sperm cell can fertilize an egg?
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c. Describe internal fertilization in humans.
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Eventually, the mass of cells becomes a hollow ball or blastula. The central, fluid-filled area is the blastocoel.
While the blastula is a single layer of cells, the process of gastrulation produces a gastrula with three cell layers:
an inner endoderm, an outer ectoderm, and a middle mesoderm. Each of these germinal layers are important to
later development. The endoderm, for example, develops into the lining of the gut while the ectoderm becomes
part of the skin and nervous system. The mesoderm develops into muscle, and some internal organs (you will
not be able to differentiate the mesodermal tissue on your slides). Gastrulation begins when cells on the surface
of the blastula begin to “push in” (much like a balloon's skin follows your finger when you push it into the side
of a balloon). The resulting hole in the side of the embryo is the blastopore and becomes either the animal's
anus or mouth
3. Continue moving the slide as needed to locate, draw, and label a blastula and an early (just pushing in)
gastrula and a late (extended with two layers) gastrula.
Blastula
Early Gastrula
Late Gastrula
a. How does a blastula differ from a gastrula?
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b. How does the size of the blastula compare to the early and late gastrula, how can you explain this?
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c. As the cells of the gastrula push in they form a channel (called the archenteron), what do you think this
channel will become, and why?
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Conclusion and Regents Questions
A. Describe the processes of fertilization, cleavage, and differentiation in humans.
B. Describe the formation of an organisms body systems from a zygote.
C. Describe how mitosis and meiosis are involved in embryonic development
B. Please process the diagrams and text of The Living Environment Regents questions, and then process the answer choices.
1. The diagram and chart below represent some of the changes a zygote undergoes during its development.
The processes that are most directly responsible for these changes are
(1) sorting and recombination of genetic information
(2) mitosis and differentiation
(3) meiosis and adaptation
(4) fertilization and cycling of materials
2. The diagram below represents a process that occurs during human reproduction.
The process represented by the arrow will ensure that the
(1) zygote contains a complete set of genetic information
(2) gametes contain a complete set of genetic information
(3) zygote contains half of the genetic information
(4) gametes contain half of the genetic information
3. The development of an embryo is represented in the diagram below.
These changes in the form of the embryo are a direct result of
(1) uncontrolled cell division and mutations
(2) differentiation and growth
(3) antibodies and antigens inherited from the father
(4) meiosis and fertilization
4. The diagram below represents stages in the processes of reproduction and development in an animal.
Cells containing only half of the genetic information characteristic of this species are found at
(1) A
(2) B
(3) C
(4) D
5. Sexual reproduction involves the processes listed below.
Processes
A. Differentiation
B. Fertilization
C. Gamete production
D. Mitosis
Which sequence represents the order in which these processes occur?
(1) A B C D
(2) B A C D
(3) C B D A
(4) D B C A