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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? __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ b. How come only one sperm cell can fertilize an egg? __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ c. Describe internal fertilization in humans. __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ 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? __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ b. How does the size of the blastula compare to the early and late gastrula, how can you explain this? __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ 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? __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ 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