Name__________________________
Biology 211 Introductory Molecular and Cell Biology
Animal Development Laboratory
Objectives:
1. Identify and compare the morula, blastula, and gastrula developmental stages
in the sea star and frog.
2. Name the three germ layers and the major organs that develop from each.
Reading: Campbell et al. Chap. 47
Introduction:
The early development of animals is similar, regardless of the species.
The fertilized egg, or zygote, undergoes successive divisions by cleavage,
forming a ball of cells called a morula, and then a hollow ball of cells called a
blastula. The fluid-filled cavity of the blastula is the blastocoel. Later, some of
the surface cells fold inward or invaginate, forming a gastrula.
The outer layer of the gastrula is called the ectoderm, and the inner layer
is the endoderm. Between these layers, a middle layer, or mesoderm, arises.
All later development can be associated with the three germ layers that give rise
to different tissues during development:
1) The ectoderm forms the skin
2) The endoderm forms the digestive system
3) The mesoderm gives rise to the circulatory, muscular, and skeletal systems
and to connective tissue.
Development requires growth, differentiation, and morphogenesis.
Growth occurs when cells divide, get larger, and divide again.
Differentiation occurs when cells become specialized in structure and function.
Morphogenesis occurs when body parts become shaped and patterned into a
certain form.
Induction is one means of organizing development. The part of the embryo that
induces the formation of an adjacent organ is said to be an organizer and is
believed to carry out its function by the release of one or more chemical
substances.
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Sea Star Development: Procedure
Sea stars and sea urchins are useful for illustrating the stages of early
development for multicellular animals. Sea stars develop in an aquatic
environment and develop quickly into a larva that can feed itself. Examine
whole-mount microscope slides of stained sea star embryos at the stages of
development listed below:
1. Unfertilized egg: Observe the large nucleus and the darkly staining
nucleolus. The plasma membrane surrounds the cytoplasm, which contains a
small amount of yolk. After fertilization, a fertilization membrane can be seen
outside the plasma membrane, and the distinct nucleus disappears.
2. Cleavage: Successive division of the embryo results in a two-cell, four-cell,
eight-cell, sixteen-cell, and finally, a many-celled stage. Cleavage occurs without
an accompanying increase in size.
3. Morula: The morula is a ball of cells that is about the same size as the
original zygote.
4. Blastula: The large number of cells in the morula rearrange to form a
blastula, a single-layered ball with a fluid-filled cavity, called the blastocoel, in the
middle.
5. Early gastrula: The cells of the blastula fold inward to form a two-layered
gastrula. The cavity produced by the infolded layer of cells is the archenterons,
or primitive gut, which has an opening to the outside called the blastopore. The
outer layer of cells is the ectoderm, and the inner layer is the endoderm.
6. Late gastrula: As development continues, two pouches (the coelomic sacs)
form by outpocketing from the endoderm surrounding the gut. These pouches
become part of the coelom (body cavity), and the walls of the lateral pouches
become the third germ layer, the mesoderm.
Use a pencil to draw the stages of development in the areas below. Use a ruler
or straight edge to make labeling lines to the right of each drawing.
Unfertilized egg
Label: plasma membrane, nucleus, nucleolus, cytoplasm
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Two-cell stage
Label: fertilization membrane
Four-cell stage
Morula
Blastula
Label: Blastocoel
Early gastrula
Label: Blastocoel, blastopore, archenteron
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Frog Development: Procedure:
Examine preserved frog embryos at various stages of development and identify
the following stages:
1. Fertilized egg: Frog eggs are partially pigmented. The black side contains
very little yolk (which contains nutrients) and is called the animal pole. The
unpigmented, yolky side is called the vegetal pole.
2. Morula: Cleavage begins at the animal pole. The first two cell divisions are
polar. The third division is horizontal or equatorial between the poles. Cleavage
continues until there is a morula. Notice that the cells of the animal pole are
smaller and more numerous than those of the vegetal pole. The yolk-laden cells
of the vegetal pole are slower to divide than those of the animal pole.
3. Blastula: The blastula is a hollow ball of cells. The blastocoel (fluid-filled
cavity) is only found at the animal pole. The yolk-laden cells of the vegetal pole
do not help in the formation of the blastocoel.
4. Early gastrula: Gastrulation is recognized by the presence of a blastopore,
where cells are invaginating. The blastopore later takes on a circular shape, but
is plugged by yolk cells that do not invaginate rapidly.
Drawing of frog gastrula
Label: yolk plug, animal pole, vegetal pole
Compare the frog gastrula to that of the sea star.
5. Late gastrula: The moderate amount of yolk also influences the formation of
the mesoderm. This germ layer develops by invagination of cells at the lateral
and ventral lips of the blastopore.
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6. Neurula: During neurulation in the frog, two folds of ectoderm grow upward
as the neural folds with a groove between them. The flat layer of ectoderm
between them is the neural plate. The tube resulting from closure of the folds in
the neural tube, which will become the spinal cord and brain.
Germ layers
In the table below, list the three germ layers and the major organs that develop
from each.
Germ layers
Organs/systems that develop from
germ layer
1.
2.
3.
Source of Laboratory:
Mader, S. (1998) Biology Laboratory Manual. WCB/McGraw Hill, Boston, MA.
Assignment (20 pts.) Complete the drawings, according to the instructions
given, and fill in the boxes on the handout and hand in by the end of the
laboratory period.
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