Download Lab Title: Embryonic development in Starfish Purpose: To observe

Lab Title: Embryonic development in Starfish
Purpose: To observe stages of development in a starfish embryo.
Background: See next page - Embryonic development
Materials: Prepared slides of Starfish embryology, digital compound microscopes, Motic
software, and computer with internet access.
Procedure: View and save images of the stages in order # 1-10. Save in jpg format with the
10 images fitting on no more that two pages total in a word document. Label parts of cells as
seen in references below and on next sheet and internet sites listed below:
http://www.uoguelph.ca/zoology/devobio/210labs/cleavage1.html
http://www.uoguelph.ca/zoology/devobio/210labs/gastrulation1.html
Slide to view and capture images. Capture image of ONE embryo only. Be sure to identify
the stage and the power of the microscope, along with parts of embryo visible.
The prepared slides provided show the different stages in starfish development from the
unfertilized egg to the early larval stage. The material has been stained for increased clarity.
1. Unfertilized egg: The unfertilized egg is a single large cell, which in your preparation may
show a distinct nucleus and prominent nucleolus. The fertilized egg has a well-defined cell
membrane, but has no additional membrane surrounding it.
2. Zygote: or fertilized egg The fertilized egg can be distinguished by the presence of a
fertilization membrane surrounding the cell. Also, the nucleus in a fertilized egg is quite
indistinct, if seen at all, and the nucleolus has disappeared.
3. and 4. Early cleavage: Find two-cell and four-cell stages. Note that the appearance of the
embryo in each case will depend on its orientation with respect to the surface of the slide.
Some embryos may be seen in end view, other in a side view at various angles.
5. Blastula: 64-cell will usually show a central cavity, the blastocoel.
6, 7, and 8: Early, Mid and Late Gastrula: In the early gastrula cells have just begun to push
in from one end. In the middle gastrula stage, cells have pushed in sufficiently to produce an
archenteron, opening to the outside by means of a wide blastopore. The formation of the
archenteron is completed in the late gastrula.
9 and 10: Larva and young starfish: Soon after gastrulation the embryo develops into a freeswimming larva. The larval stages are shown in figure 11.1 of your lab manual; they may or
may not be seen or your slide. The first larval stage is called the bipinnaria larva. This is
transformed into a brachiolaria larva, characterized by its many, long "arms." The young
starfish, with its 5-radiate symmetry, develops from the brachiolaria stage.
Conclusion:
Describe the process of embryological development. Begin with the unfertilized egg and
discuss, in detail, the changes that occur from slide to slide observed today. Use appropriate
terms by referencing your background handout. From what parts of the gastrula do the ecto,
meso and endo derms form? What difficulties did you have in the lab today? Discuss one
idea for further study in embryological development?
Embryo Development Reference: (Use to help label your pictures and for conclusion)
In multicellular organisms that reproduce sexually, fertilization of the egg by the sperm
produces the zygote, which marks the beginning of the new individual. During the period of
development of the single-celled zygote until the hatching or birth of the multicellular
individual, the new organism is referred to as the embryo. The study of the development of
the embryo is called embryology.
Soon after fertilization, the zygote begins cleavage. This process consists of successive cell
divisions, in vertical and horizontal planes, so that each division doubles the previous number
of cells.
After a few such divisions, the embryo consists of a ball of many cells. At first, the cells are
clustered together without any distinct space or cavity between them, and the embryo
somewhat resembles a mulberry. It is therefore called the morula (Latin morum = mulberry).
In most cases, however, the cells soon rearrange themselves to form a definite ball around a
central cavity. This spherical, hollow ball of cells is called the blastula, and its central cavity the
blastocoel.
Further development of the blastula results in the formation of a gastrula, a stage
characterized by the appearance within the embryo of a tubular or sac-like structure called the
archenteron (= primitive gut). As the name suggests, this tube is destined to become the
digestive tract. At this early stage, the archenteron has only one opening to the outside, which
is called the blastopore.
The process of formation of the gastrula is called gastrulation. Gastrulation is accomplished
by the pushing in of cells from one end of the blastula. It may be likened to the pushing in of
an inflated balloon from one end with your finger. However, in the embryo, cells migrate from
the outside to the interior.
Because of the formation of the archenteron, the embryo now consists of two walls. The
outside wall, which was also present in the blastula, is now called the ectoderm; the wall of the
archenteron, which is in the interior, is called the endoderm. The space between the two is
what remains of the original blastocoel.
In the majority of animals a third layer of cells now arises and occupies much of the space
between the ectoderm and the endoderm (thus further reducing, or in some cases even
completely obliterating, the blastocoel). This middle layer is called the mesoderm.
The ectoderm, endoderm and mesoderm are called the primary germinal layers. Once these
layers are established, tissue differentiation and organ formation proceed rapidly so as to give
the embryo its characteristic shape and form. The primary germinal layers are specialized in
that each of them has the potential to give rise to only certain specific structures.
The structures normally derived from each layer are as follows:
Ectoderm
Outer layers (epidermis) of the skin, including cuticle, scales, hair, feathers, nails, claws, hoofs,
teeth, and skin glands; lining of mouth and nervous system with external sensory receptors.
Mesoderm
Lining of the body cavities, muscles, connective tissue including cartilage and bone, vascular
tissue - heart, blood vessels and blood, excretory and reproductive organs.
Endoderm
Lining of digestive tract from pharynx to anus, liver, pancreas
(In land vertebrates) lining of the respiratory passages and lungs